* [PATCH] c++: -Wdangling-reference with reference wrapper [PR107532]
@ 2023-01-18 17:52 Marek Polacek
2023-01-18 21:07 ` Jason Merrill
0 siblings, 1 reply; 17+ messages in thread
From: Marek Polacek @ 2023-01-18 17:52 UTC (permalink / raw)
To: Jason Merrill, GCC Patches
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
So I figured that perhaps we want to look at the object we're invoking
the member function(s) on and see if that is a temporary, as in, don't
warn about
const Plane & meta = fm.planes().inner();
but do warn about
const Plane & meta = FrameMetadata().planes().inner();
It's ugly, but better than asking users to add #pragmas into their code.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (do_warn_dangling_reference): Don't warn when the
object member functions are invoked on is not a temporary.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
---
gcc/cp/call.cc | 33 +++++++-
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
2 files changed, 109 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 0780b5840a3..43e65c3dffb 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13850,7 +13850,38 @@ do_warn_dangling_reference (tree expr)
if (TREE_CODE (arg) == ADDR_EXPR)
arg = TREE_OPERAND (arg, 0);
if (expr_represents_temporary_p (arg))
- return expr;
+ {
+ /* An ugly attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Don't warn about s.a().b() but do warn about S().a().b(),
+ supposing that the member function is returning a reference
+ to a subobject of the (non-temporary) object. */
+ if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+ && !DECL_OVERLOADED_OPERATOR_P (fndecl)
+ && i == 0)
+ {
+ tree t = arg;
+ while (handled_component_p (t))
+ t = TREE_OPERAND (t, 0);
+ t = TARGET_EXPR_INITIAL (arg);
+ /* Quite likely we don't have a chain of member functions
+ (like a().b().c()). */
+ if (TREE_CODE (t) != CALL_EXPR)
+ return expr;
+ /* Walk the call chain to the original object and see if
+ it was a temporary. */
+ do
+ t = tree_strip_nop_conversions (CALL_EXPR_ARG (t, 0));
+ while (TREE_CODE (t) == CALL_EXPR);
+ /* If the object argument is &TARGET_EXPR<>, we've started
+ off the chain with a temporary and we want to warn. */
+ if (TREE_CODE (t) == ADDR_EXPR)
+ t = TREE_OPERAND (t, 0);
+ if (!expr_represents_temporary_p (t))
+ break;
+ }
+ return expr;
+ }
/* Don't warn about member function like:
std::any a(...);
S& s = a.emplace<S>({0}, 0);
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..32280f3e282
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner(); // { dg-warning "dangling reference" }
+ (void) d8;
+}
base-commit: c6a011119bfa038ccbfc9f123ede14a3d6237fab
--
2.39.0
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-01-18 17:52 [PATCH] c++: -Wdangling-reference with reference wrapper [PR107532] Marek Polacek
@ 2023-01-18 21:07 ` Jason Merrill
2023-01-19 1:13 ` [PATCH v2] " Marek Polacek
0 siblings, 1 reply; 17+ messages in thread
From: Jason Merrill @ 2023-01-18 21:07 UTC (permalink / raw)
To: Marek Polacek, GCC Patches
On 1/18/23 12:52, Marek Polacek wrote:
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> So I figured that perhaps we want to look at the object we're invoking
> the member function(s) on and see if that is a temporary, as in, don't
> warn about
>
> const Plane & meta = fm.planes().inner();
>
> but do warn about
>
> const Plane & meta = FrameMetadata().planes().inner();
>
> It's ugly, but better than asking users to add #pragmas into their code.
Hmm, that doesn't seem right; the former is only OK because Ref is in
fact a reference-like type. If planes() returned a class that held
data, we would want to warn.
In this case, we might recognize the reference-like class because it has
a reference member and a constructor taking the same reference type.
That wouldn't help with std::reference_wrapper or std::ref_view because
they have pointer members instead of references, but perhaps loosening
the check to include that case would make sense?
Jason
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (do_warn_dangling_reference): Don't warn when the
> object member functions are invoked on is not a temporary.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> ---
> gcc/cp/call.cc | 33 +++++++-
> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
> 2 files changed, 109 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 0780b5840a3..43e65c3dffb 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13850,7 +13850,38 @@ do_warn_dangling_reference (tree expr)
> if (TREE_CODE (arg) == ADDR_EXPR)
> arg = TREE_OPERAND (arg, 0);
> if (expr_represents_temporary_p (arg))
> - return expr;
> + {
> + /* An ugly attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + Don't warn about s.a().b() but do warn about S().a().b(),
> + supposing that the member function is returning a reference
> + to a subobject of the (non-temporary) object. */
> + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> + && i == 0)
> + {
> + tree t = arg;
> + while (handled_component_p (t))
> + t = TREE_OPERAND (t, 0);
> + t = TARGET_EXPR_INITIAL (arg);
> + /* Quite likely we don't have a chain of member functions
> + (like a().b().c()). */
> + if (TREE_CODE (t) != CALL_EXPR)
> + return expr;
> + /* Walk the call chain to the original object and see if
> + it was a temporary. */
> + do
> + t = tree_strip_nop_conversions (CALL_EXPR_ARG (t, 0));
> + while (TREE_CODE (t) == CALL_EXPR);
> + /* If the object argument is &TARGET_EXPR<>, we've started
> + off the chain with a temporary and we want to warn. */
> + if (TREE_CODE (t) == ADDR_EXPR)
> + t = TREE_OPERAND (t, 0);
> + if (!expr_represents_temporary_p (t))
> + break;
> + }
> + return expr;
> + }
> /* Don't warn about member function like:
> std::any a(...);
> S& s = a.emplace<S>({0}, 0);
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..32280f3e282
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner(); // { dg-warning "dangling reference" }
> + (void) d8;
> +}
>
> base-commit: c6a011119bfa038ccbfc9f123ede14a3d6237fab
^ permalink raw reply [flat|nested] 17+ messages in thread
* [PATCH v2] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-01-18 21:07 ` Jason Merrill
@ 2023-01-19 1:13 ` Marek Polacek
2023-01-19 18:02 ` Jason Merrill
0 siblings, 1 reply; 17+ messages in thread
From: Marek Polacek @ 2023-01-19 1:13 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> On 1/18/23 12:52, Marek Polacek wrote:
> > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > some grief. The code in question uses a reference wrapper with a member
> > function returning a reference to a subobject of a non-temporary object:
> >
> > const Plane & meta = fm.planes().inner();
> >
> > I've tried a few approaches, e.g., checking that the member function's
> > return type is the same as the type of the enclosing class (which is
> > the case for member functions returning *this), but that then breaks
> > Wdangling-reference4.C with std::optional<std::string>.
> >
> > So I figured that perhaps we want to look at the object we're invoking
> > the member function(s) on and see if that is a temporary, as in, don't
> > warn about
> >
> > const Plane & meta = fm.planes().inner();
> >
> > but do warn about
> >
> > const Plane & meta = FrameMetadata().planes().inner();
> >
> > It's ugly, but better than asking users to add #pragmas into their code.
>
> Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
> reference-like type. If planes() returned a class that held data, we would
> want to warn.
Sure, it's always some kind of tradeoff with warnings :/.
> In this case, we might recognize the reference-like class because it has a
> reference member and a constructor taking the same reference type.
That occurred to me too, but then I found out that std::reference_wrapper
actually uses T*, not T&, as you say. But here's a patch to do that
(I hope).
> That wouldn't help with std::reference_wrapper or std::ref_view because they
> have pointer members instead of references, but perhaps loosening the check
> to include that case would make sense?
Sorry, I don't understand what you mean by loosening the check. I could
hardcode std::reference_wrapper and std::ref_view but I don't think that's
what you meant. Surely I cannot _not_ warn for any class that contains a
T*.
Here's the patch so that we have some actual code to discuss... Thanks.
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
Perhaps we want to look at the member function's enclosing class
to see if it's a reference wrapper class (meaning, has a reference
member and a constructor taking the same reference type) and don't
warn if so, supposing that the member function returns a reference
to a non-temporary object.
It's ugly, but better than asking users to add #pragmas into their code.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (do_warn_dangling_reference): Don't warn when the
member function comes from a reference wrapper class.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
---
gcc/cp/call.cc | 32 ++++++++
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
2 files changed, 109 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 0780b5840a3..b0670a21240 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13832,6 +13832,38 @@ do_warn_dangling_reference (tree expr)
if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
return NULL_TREE;
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ If the enclosing class is a reference-like class, that is, has
+ a reference member and a constructor taking the same reference type,
+ we suppose that the member function is returning a reference
+ to a non-temporary object. */
+ if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+ && !DECL_OVERLOADED_OPERATOR_P (fndecl))
+ {
+ tree ctx = CP_DECL_CONTEXT (fndecl);
+ for (tree fields = TYPE_FIELDS (ctx);
+ fields;
+ fields = DECL_CHAIN (fields))
+ {
+ if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+ continue;
+ tree type = TREE_TYPE (fields);
+ if (!TYPE_REF_P (type))
+ continue;
+ /* OK, the field is a reference member. Do we have
+ a constructor taking its type? */
+ for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctx)))
+ {
+ tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+ if (args
+ && same_type_p (TREE_VALUE (args), type)
+ && TREE_CHAIN (args) == void_list_node)
+ return NULL_TREE;
+ }
+ }
+ }
+
/* Here we're looking to see if any of the arguments is a temporary
initializing a reference parameter. */
for (int i = 0; i < call_expr_nargs (expr); ++i)
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..4d585891fae
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner();
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner();
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner();
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner();
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
base-commit: 8e2c6e7b426b6c9c13076208b2e176d4aa1432f1
--
2.39.0
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v2] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-01-19 1:13 ` [PATCH v2] " Marek Polacek
@ 2023-01-19 18:02 ` Jason Merrill
2023-01-20 2:03 ` [PATCH v3] " Marek Polacek
0 siblings, 1 reply; 17+ messages in thread
From: Jason Merrill @ 2023-01-19 18:02 UTC (permalink / raw)
To: Marek Polacek; +Cc: GCC Patches
On 1/18/23 20:13, Marek Polacek wrote:
> On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
>> On 1/18/23 12:52, Marek Polacek wrote:
>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>> some grief. The code in question uses a reference wrapper with a member
>>> function returning a reference to a subobject of a non-temporary object:
>>>
>>> const Plane & meta = fm.planes().inner();
>>>
>>> I've tried a few approaches, e.g., checking that the member function's
>>> return type is the same as the type of the enclosing class (which is
>>> the case for member functions returning *this), but that then breaks
>>> Wdangling-reference4.C with std::optional<std::string>.
>>>
>>> So I figured that perhaps we want to look at the object we're invoking
>>> the member function(s) on and see if that is a temporary, as in, don't
>>> warn about
>>>
>>> const Plane & meta = fm.planes().inner();
>>>
>>> but do warn about
>>>
>>> const Plane & meta = FrameMetadata().planes().inner();
>>>
>>> It's ugly, but better than asking users to add #pragmas into their code.
>>
>> Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
>> reference-like type. If planes() returned a class that held data, we would
>> want to warn.
>
> Sure, it's always some kind of tradeoff with warnings :/.
>
>> In this case, we might recognize the reference-like class because it has a
>> reference member and a constructor taking the same reference type.
>
> That occurred to me too, but then I found out that std::reference_wrapper
> actually uses T*, not T&, as you say. But here's a patch to do that
> (I hope).
>
>> That wouldn't help with std::reference_wrapper or std::ref_view because they
>> have pointer members instead of references, but perhaps loosening the check
>> to include that case would make sense?
>
> Sorry, I don't understand what you mean by loosening the check. I could
> hardcode std::reference_wrapper and std::ref_view but I don't think that's
> what you meant.
Indeed that's not what I meant, but as I was saying in our meeting I
think it's worth doing; the compiler has various tweaks to handle
specific standard-library classes better.
> Surely I cannot _not_ warn for any class that contains a T*.
I was thinking if a constructor takes a T& and the class has a T* that
would be close enough, though this also wouldn't handle the standard
library classes so the benefit is questionable.
> Here's the patch so that we have some actual code to discuss... Thanks.
>
> -- >8 --
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> Perhaps we want to look at the member function's enclosing class
> to see if it's a reference wrapper class (meaning, has a reference
> member and a constructor taking the same reference type) and don't
> warn if so, supposing that the member function returns a reference
> to a non-temporary object.
>
> It's ugly, but better than asking users to add #pragmas into their code.
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (do_warn_dangling_reference): Don't warn when the
> member function comes from a reference wrapper class.
Let's factor the new code out into e.g. reference_like_class_p
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> ---
> gcc/cp/call.cc | 32 ++++++++
> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
> 2 files changed, 109 insertions(+)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 0780b5840a3..b0670a21240 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13832,6 +13832,38 @@ do_warn_dangling_reference (tree expr)
> if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> return NULL_TREE;
>
> + /* An attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + If the enclosing class is a reference-like class, that is, has
> + a reference member and a constructor taking the same reference type,
> + we suppose that the member function is returning a reference
> + to a non-temporary object. */
> + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> + && !DECL_OVERLOADED_OPERATOR_P (fndecl))
> + {
> + tree ctx = CP_DECL_CONTEXT (fndecl);
> + for (tree fields = TYPE_FIELDS (ctx);
> + fields;
> + fields = DECL_CHAIN (fields))
> + {
> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> + continue;
> + tree type = TREE_TYPE (fields);
> + if (!TYPE_REF_P (type))
> + continue;
> + /* OK, the field is a reference member. Do we have
> + a constructor taking its type? */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctx)))
> + {
> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> + if (args
> + && same_type_p (TREE_VALUE (args), type)
> + && TREE_CHAIN (args) == void_list_node)
> + return NULL_TREE;
> + }
> + }
> + }
> +
> /* Here we're looking to see if any of the arguments is a temporary
> initializing a reference parameter. */
> for (int i = 0; i < call_expr_nargs (expr); ++i)
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..4d585891fae
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner();
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner();
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner();
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner();
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner();
> + (void) d8;
> +}
>
> base-commit: 8e2c6e7b426b6c9c13076208b2e176d4aa1432f1
^ permalink raw reply [flat|nested] 17+ messages in thread
* [PATCH v3] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-01-19 18:02 ` Jason Merrill
@ 2023-01-20 2:03 ` Marek Polacek
2023-01-20 20:19 ` Jason Merrill
0 siblings, 1 reply; 17+ messages in thread
From: Marek Polacek @ 2023-01-20 2:03 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
> On 1/18/23 20:13, Marek Polacek wrote:
> > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> > > On 1/18/23 12:52, Marek Polacek wrote:
> > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > some grief. The code in question uses a reference wrapper with a member
> > > > function returning a reference to a subobject of a non-temporary object:
> > > >
> > > > const Plane & meta = fm.planes().inner();
> > > >
> > > > I've tried a few approaches, e.g., checking that the member function's
> > > > return type is the same as the type of the enclosing class (which is
> > > > the case for member functions returning *this), but that then breaks
> > > > Wdangling-reference4.C with std::optional<std::string>.
> > > >
> > > > So I figured that perhaps we want to look at the object we're invoking
> > > > the member function(s) on and see if that is a temporary, as in, don't
> > > > warn about
> > > >
> > > > const Plane & meta = fm.planes().inner();
> > > >
> > > > but do warn about
> > > >
> > > > const Plane & meta = FrameMetadata().planes().inner();
> > > >
> > > > It's ugly, but better than asking users to add #pragmas into their code.
> > >
> > > Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
> > > reference-like type. If planes() returned a class that held data, we would
> > > want to warn.
> >
> > Sure, it's always some kind of tradeoff with warnings :/.
> > > In this case, we might recognize the reference-like class because it has a
> > > reference member and a constructor taking the same reference type.
> >
> > That occurred to me too, but then I found out that std::reference_wrapper
> > actually uses T*, not T&, as you say. But here's a patch to do that
> > (I hope).
> > > That wouldn't help with std::reference_wrapper or std::ref_view because they
> > > have pointer members instead of references, but perhaps loosening the check
> > > to include that case would make sense?
> >
> > Sorry, I don't understand what you mean by loosening the check. I could
> > hardcode std::reference_wrapper and std::ref_view but I don't think that's
> > what you meant.
>
> Indeed that's not what I meant, but as I was saying in our meeting I think
> it's worth doing; the compiler has various tweaks to handle specific
> standard-library classes better.
Okay, done in the patch below. Except that I'm not including a test for
std::ranges::ref_view because I don't really know how that works.
> > Surely I cannot _not_ warn for any class that contains a T*.
>
> I was thinking if a constructor takes a T& and the class has a T* that would
> be close enough, though this also wouldn't handle the standard library
> classes so the benefit is questionable.
>
> > Here's the patch so that we have some actual code to discuss... Thanks.
> >
> > -- >8 --
> > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > some grief. The code in question uses a reference wrapper with a member
> > function returning a reference to a subobject of a non-temporary object:
> >
> > const Plane & meta = fm.planes().inner();
> >
> > I've tried a few approaches, e.g., checking that the member function's
> > return type is the same as the type of the enclosing class (which is
> > the case for member functions returning *this), but that then breaks
> > Wdangling-reference4.C with std::optional<std::string>.
> >
> > Perhaps we want to look at the member function's enclosing class
> > to see if it's a reference wrapper class (meaning, has a reference
> > member and a constructor taking the same reference type) and don't
> > warn if so, supposing that the member function returns a reference
> > to a non-temporary object.
> >
> > It's ugly, but better than asking users to add #pragmas into their code.
> >
> > PR c++/107532
> >
> > gcc/cp/ChangeLog:
> >
> > * call.cc (do_warn_dangling_reference): Don't warn when the
> > member function comes from a reference wrapper class.
>
> Let's factor the new code out into e.g. reference_like_class_p
Done. Thanks,
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
Perhaps we want to look at the member function's enclosing class
to see if it's a reference wrapper class (meaning, has a reference
member and a constructor taking the same reference type, or is
std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
supposing that the member function returns a reference to a non-temporary
object.
It's ugly, but better than asking users to add #pragmas into their code.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Don't warn when the member function comes
from a reference_like_class_p.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
* g++.dg/warn/Wdangling-reference9.C: New test.
---
gcc/cp/call.cc | 48 ++++++++++++
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 +++++
3 files changed, 146 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 991730713e6..672722998ee 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
return true;
}
+/* Return true if a class CTYPE is either std::reference_wrapper or
+ std::ref_view, or a reference wrapper class. We consider a class
+ a reference wrapper class if it has a reference member and a
+ constructor taking the same reference type. */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+ if (decl_in_std_namespace_p (tdecl))
+ {
+ tree name = DECL_NAME (tdecl);
+ return (name
+ && (id_equal (name, "reference_wrapper")
+ || id_equal (name, "ref_view")));
+ }
+ for (tree fields = TYPE_FIELDS (ctype);
+ fields;
+ fields = DECL_CHAIN (fields))
+ {
+ if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+ continue;
+ tree type = TREE_TYPE (fields);
+ if (!TYPE_REF_P (type))
+ continue;
+ /* OK, the field is a reference member. Do we have a constructor
+ taking its type? */
+ for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
+ {
+ tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+ if (args
+ && same_type_p (TREE_VALUE (args), type)
+ && TREE_CHAIN (args) == void_list_node)
+ return true;
+ }
+ }
+ return false;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
return NULL_TREE;
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Here we suppose that a member function of such a reference
+ wrapper class returns a reference to a non-temporary object. */
+ if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+ && !DECL_OVERLOADED_OPERATOR_P (fndecl)
+ && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
+ return NULL_TREE;
+
/* Here we're looking to see if any of the arguments is a temporary
initializing a reference parameter. */
for (int i = 0; i < call_expr_nargs (expr); ++i)
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..4d585891fae
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner();
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner();
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner();
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner();
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..15c1f6b9dd2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+ std::reference_wrapper<const X> wrapit() const { return x; }
+ X x;
+};
+
+void
+g (const S& s)
+{
+ const auto& a1 = s.wrapit().get();
+ (void) a1;
+ const auto& a2 = S().wrapit().get();
+ (void) a2;
+}
base-commit: 0846336de56119777861e02bf68f92a6af466000
--
2.39.0
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v3] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-01-20 2:03 ` [PATCH v3] " Marek Polacek
@ 2023-01-20 20:19 ` Jason Merrill
2023-01-24 22:49 ` Marek Polacek
0 siblings, 1 reply; 17+ messages in thread
From: Jason Merrill @ 2023-01-20 20:19 UTC (permalink / raw)
To: Marek Polacek; +Cc: GCC Patches
On 1/19/23 21:03, Marek Polacek wrote:
> On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
>> On 1/18/23 20:13, Marek Polacek wrote:
>>> On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
>>>> On 1/18/23 12:52, Marek Polacek wrote:
>>>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>>>> some grief. The code in question uses a reference wrapper with a member
>>>>> function returning a reference to a subobject of a non-temporary object:
>>>>>
>>>>> const Plane & meta = fm.planes().inner();
>>>>>
>>>>> I've tried a few approaches, e.g., checking that the member function's
>>>>> return type is the same as the type of the enclosing class (which is
>>>>> the case for member functions returning *this), but that then breaks
>>>>> Wdangling-reference4.C with std::optional<std::string>.
>>>>>
>>>>> So I figured that perhaps we want to look at the object we're invoking
>>>>> the member function(s) on and see if that is a temporary, as in, don't
>>>>> warn about
>>>>>
>>>>> const Plane & meta = fm.planes().inner();
>>>>>
>>>>> but do warn about
>>>>>
>>>>> const Plane & meta = FrameMetadata().planes().inner();
>>>>>
>>>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>>
>>>> Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
>>>> reference-like type. If planes() returned a class that held data, we would
>>>> want to warn.
>>>
>>> Sure, it's always some kind of tradeoff with warnings :/.
>>>> In this case, we might recognize the reference-like class because it has a
>>>> reference member and a constructor taking the same reference type.
>>>
>>> That occurred to me too, but then I found out that std::reference_wrapper
>>> actually uses T*, not T&, as you say. But here's a patch to do that
>>> (I hope).
>>>> That wouldn't help with std::reference_wrapper or std::ref_view because they
>>>> have pointer members instead of references, but perhaps loosening the check
>>>> to include that case would make sense?
>>>
>>> Sorry, I don't understand what you mean by loosening the check. I could
>>> hardcode std::reference_wrapper and std::ref_view but I don't think that's
>>> what you meant.
>>
>> Indeed that's not what I meant, but as I was saying in our meeting I think
>> it's worth doing; the compiler has various tweaks to handle specific
>> standard-library classes better.
>
> Okay, done in the patch below. Except that I'm not including a test for
> std::ranges::ref_view because I don't really know how that works.
>
>>> Surely I cannot _not_ warn for any class that contains a T*.
>>
>> I was thinking if a constructor takes a T& and the class has a T* that would
>> be close enough, though this also wouldn't handle the standard library
>> classes so the benefit is questionable.
>>
>>> Here's the patch so that we have some actual code to discuss... Thanks.
>>>
>>> -- >8 --
>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>> some grief. The code in question uses a reference wrapper with a member
>>> function returning a reference to a subobject of a non-temporary object:
>>>
>>> const Plane & meta = fm.planes().inner();
>>>
>>> I've tried a few approaches, e.g., checking that the member function's
>>> return type is the same as the type of the enclosing class (which is
>>> the case for member functions returning *this), but that then breaks
>>> Wdangling-reference4.C with std::optional<std::string>.
>>>
>>> Perhaps we want to look at the member function's enclosing class
>>> to see if it's a reference wrapper class (meaning, has a reference
>>> member and a constructor taking the same reference type) and don't
>>> warn if so, supposing that the member function returns a reference
>>> to a non-temporary object.
>>>
>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>
>>> PR c++/107532
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> * call.cc (do_warn_dangling_reference): Don't warn when the
>>> member function comes from a reference wrapper class.
>>
>> Let's factor the new code out into e.g. reference_like_class_p
>
> Done. Thanks,
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> Perhaps we want to look at the member function's enclosing class
> to see if it's a reference wrapper class (meaning, has a reference
> member and a constructor taking the same reference type, or is
> std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
> supposing that the member function returns a reference to a non-temporary
> object.
>
> It's ugly, but better than asking users to add #pragmas into their code.
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (reference_like_class_p): New.
> (do_warn_dangling_reference): Don't warn when the member function comes
> from a reference_like_class_p.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> * g++.dg/warn/Wdangling-reference9.C: New test.
> ---
> gcc/cp/call.cc | 48 ++++++++++++
> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
> .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
> 3 files changed, 146 insertions(+)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 991730713e6..672722998ee 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> return true;
> }
>
> +/* Return true if a class CTYPE is either std::reference_wrapper or
> + std::ref_view, or a reference wrapper class. We consider a class
> + a reference wrapper class if it has a reference member and a
> + constructor taking the same reference type. */
> +
> +static bool
> +reference_like_class_p (tree ctype)
> +{
> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> + if (decl_in_std_namespace_p (tdecl))
> + {
> + tree name = DECL_NAME (tdecl);
> + return (name
> + && (id_equal (name, "reference_wrapper")
> + || id_equal (name, "ref_view")));
> + }
> + for (tree fields = TYPE_FIELDS (ctype);
> + fields;
> + fields = DECL_CHAIN (fields))
> + {
> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> + continue;
> + tree type = TREE_TYPE (fields);
> + if (!TYPE_REF_P (type))
> + continue;
> + /* OK, the field is a reference member. Do we have a constructor
> + taking its type? */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> + {
> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> + if (args
> + && same_type_p (TREE_VALUE (args), type)
> + && TREE_CHAIN (args) == void_list_node)
> + return true;
> + }
> + }
> + return false;
> +}
> +
> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> that initializes the LHS (and at least one of its arguments represents
> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
> if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> return NULL_TREE;
>
> + /* An attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + Here we suppose that a member function of such a reference
> + wrapper class returns a reference to a non-temporary object. */
> + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
Ah, in this case I was thinking rather than return we would want to look
through to the initializer of the reference wrapper, and warn if that's
a temporary, so we can catch the *2 cases in your tests.
So, treating ref-like classes as much like references as we can. Some
of your v1 patch ought to be useful in implementing this, but only
looking through one call at a time, not all of them like that patch.
> + return NULL_TREE;
> +
> /* Here we're looking to see if any of the arguments is a temporary
> initializing a reference parameter. */
> for (int i = 0; i < call_expr_nargs (expr); ++i)
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..4d585891fae
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner();
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner();
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner();
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner();
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner();
> + (void) d8;
> +}
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> new file mode 100644
> index 00000000000..15c1f6b9dd2
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> @@ -0,0 +1,21 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <functional>
> +
> +struct X { int n; };
> +
> +struct S {
> + std::reference_wrapper<const X> wrapit() const { return x; }
> + X x;
> +};
> +
> +void
> +g (const S& s)
> +{
> + const auto& a1 = s.wrapit().get();
> + (void) a1;
> + const auto& a2 = S().wrapit().get();
> + (void) a2;
> +}
>
> base-commit: 0846336de56119777861e02bf68f92a6af466000
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v3] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-01-20 20:19 ` Jason Merrill
@ 2023-01-24 22:49 ` Marek Polacek
2023-02-06 1:25 ` Jason Merrill
0 siblings, 1 reply; 17+ messages in thread
From: Marek Polacek @ 2023-01-24 22:49 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote:
> On 1/19/23 21:03, Marek Polacek wrote:
> > On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
> > > On 1/18/23 20:13, Marek Polacek wrote:
> > > > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> > > > > On 1/18/23 12:52, Marek Polacek wrote:
> > > > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > > > some grief. The code in question uses a reference wrapper with a member
> > > > > > function returning a reference to a subobject of a non-temporary object:
> > > > > >
> > > > > > const Plane & meta = fm.planes().inner();
> > > > > >
> > > > > > I've tried a few approaches, e.g., checking that the member function's
> > > > > > return type is the same as the type of the enclosing class (which is
> > > > > > the case for member functions returning *this), but that then breaks
> > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > >
> > > > > > So I figured that perhaps we want to look at the object we're invoking
> > > > > > the member function(s) on and see if that is a temporary, as in, don't
> > > > > > warn about
> > > > > >
> > > > > > const Plane & meta = fm.planes().inner();
> > > > > >
> > > > > > but do warn about
> > > > > >
> > > > > > const Plane & meta = FrameMetadata().planes().inner();
> > > > > >
> > > > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > >
> > > > > Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
> > > > > reference-like type. If planes() returned a class that held data, we would
> > > > > want to warn.
> > > >
> > > > Sure, it's always some kind of tradeoff with warnings :/.
> > > > > In this case, we might recognize the reference-like class because it has a
> > > > > reference member and a constructor taking the same reference type.
> > > >
> > > > That occurred to me too, but then I found out that std::reference_wrapper
> > > > actually uses T*, not T&, as you say. But here's a patch to do that
> > > > (I hope).
> > > > > That wouldn't help with std::reference_wrapper or std::ref_view because they
> > > > > have pointer members instead of references, but perhaps loosening the check
> > > > > to include that case would make sense?
> > > >
> > > > Sorry, I don't understand what you mean by loosening the check. I could
> > > > hardcode std::reference_wrapper and std::ref_view but I don't think that's
> > > > what you meant.
> > >
> > > Indeed that's not what I meant, but as I was saying in our meeting I think
> > > it's worth doing; the compiler has various tweaks to handle specific
> > > standard-library classes better.
> > Okay, done in the patch below. Except that I'm not including a test for
> > std::ranges::ref_view because I don't really know how that works.
> >
> > > > Surely I cannot _not_ warn for any class that contains a T*.
> > >
> > > I was thinking if a constructor takes a T& and the class has a T* that would
> > > be close enough, though this also wouldn't handle the standard library
> > > classes so the benefit is questionable.
> > >
> > > > Here's the patch so that we have some actual code to discuss... Thanks.
> > > >
> > > > -- >8 --
> > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > some grief. The code in question uses a reference wrapper with a member
> > > > function returning a reference to a subobject of a non-temporary object:
> > > >
> > > > const Plane & meta = fm.planes().inner();
> > > >
> > > > I've tried a few approaches, e.g., checking that the member function's
> > > > return type is the same as the type of the enclosing class (which is
> > > > the case for member functions returning *this), but that then breaks
> > > > Wdangling-reference4.C with std::optional<std::string>.
> > > >
> > > > Perhaps we want to look at the member function's enclosing class
> > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > member and a constructor taking the same reference type) and don't
> > > > warn if so, supposing that the member function returns a reference
> > > > to a non-temporary object.
> > > >
> > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > >
> > > > PR c++/107532
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > > * call.cc (do_warn_dangling_reference): Don't warn when the
> > > > member function comes from a reference wrapper class.
> > >
> > > Let's factor the new code out into e.g. reference_like_class_p
> >
> > Done. Thanks,
> >
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > some grief. The code in question uses a reference wrapper with a member
> > function returning a reference to a subobject of a non-temporary object:
> >
> > const Plane & meta = fm.planes().inner();
> >
> > I've tried a few approaches, e.g., checking that the member function's
> > return type is the same as the type of the enclosing class (which is
> > the case for member functions returning *this), but that then breaks
> > Wdangling-reference4.C with std::optional<std::string>.
> >
> > Perhaps we want to look at the member function's enclosing class
> > to see if it's a reference wrapper class (meaning, has a reference
> > member and a constructor taking the same reference type, or is
> > std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
> > supposing that the member function returns a reference to a non-temporary
> > object.
> >
> > It's ugly, but better than asking users to add #pragmas into their code.
> >
> > PR c++/107532
> >
> > gcc/cp/ChangeLog:
> >
> > * call.cc (reference_like_class_p): New.
> > (do_warn_dangling_reference): Don't warn when the member function comes
> > from a reference_like_class_p.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/warn/Wdangling-reference8.C: New test.
> > * g++.dg/warn/Wdangling-reference9.C: New test.
> > ---
> > gcc/cp/call.cc | 48 ++++++++++++
> > .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
> > .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
> > 3 files changed, 146 insertions(+)
> > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> >
> > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > index 991730713e6..672722998ee 100644
> > --- a/gcc/cp/call.cc
> > +++ b/gcc/cp/call.cc
> > @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> > return true;
> > }
> > +/* Return true if a class CTYPE is either std::reference_wrapper or
> > + std::ref_view, or a reference wrapper class. We consider a class
> > + a reference wrapper class if it has a reference member and a
> > + constructor taking the same reference type. */
> > +
> > +static bool
> > +reference_like_class_p (tree ctype)
> > +{
> > + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> > + if (decl_in_std_namespace_p (tdecl))
> > + {
> > + tree name = DECL_NAME (tdecl);
> > + return (name
> > + && (id_equal (name, "reference_wrapper")
> > + || id_equal (name, "ref_view")));
> > + }
> > + for (tree fields = TYPE_FIELDS (ctype);
> > + fields;
> > + fields = DECL_CHAIN (fields))
> > + {
> > + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> > + continue;
> > + tree type = TREE_TYPE (fields);
> > + if (!TYPE_REF_P (type))
> > + continue;
> > + /* OK, the field is a reference member. Do we have a constructor
> > + taking its type? */
> > + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> > + {
> > + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> > + if (args
> > + && same_type_p (TREE_VALUE (args), type)
> > + && TREE_CHAIN (args) == void_list_node)
> > + return true;
> > + }
> > + }
> > + return false;
> > +}
> > +
> > /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> > that initializes the LHS (and at least one of its arguments represents
> > a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> > @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
> > if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> > return NULL_TREE;
> > + /* An attempt to reduce the number of -Wdangling-reference
> > + false positives concerning reference wrappers (c++/107532).
> > + Here we suppose that a member function of such a reference
> > + wrapper class returns a reference to a non-temporary object. */
> > + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> > + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
>
> Ah, in this case I was thinking rather than return we would want to look
> through to the initializer of the reference wrapper, and warn if that's a
> temporary, so we can catch the *2 cases in your tests.
>
> So, treating ref-like classes as much like references as we can. Some of
> your v1 patch ought to be useful in implementing this, but only looking
> through one call at a time, not all of them like that patch.
Maybe this one, then? I still have to loop through the calls though; EXPR in
do_warn_dangling_reference can be e.g.
Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>)
or
Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>)
and we want to warn only about the latter, but that means that I need to
look into the nested call 'planes' to see if the initializer was a temporary.
With this, we warn for the *2 cases too.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
Perhaps we want to look at the member function's enclosing class
to see if it's a reference wrapper class (meaning, has a reference
member and a constructor taking the same reference type, or is
std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
supposing that the member function returns a reference to a non-temporary
object.
It's ugly, but better than asking users to add #pragmas into their code.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Don't warn when the member function comes
from a reference_like_class_p.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
* g++.dg/warn/Wdangling-reference9.C: New test.
---
gcc/cp/call.cc | 70 ++++++++++++++++-
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 +++++
3 files changed, 167 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 5715a7cd1de..137870670e7 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
return true;
}
+/* Return true if a class CTYPE is either std::reference_wrapper or
+ std::ref_view, or a reference wrapper class. We consider a class
+ a reference wrapper class if it has a reference member and a
+ constructor taking the same reference type. */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+ if (decl_in_std_namespace_p (tdecl))
+ {
+ tree name = DECL_NAME (tdecl);
+ return (name
+ && (id_equal (name, "reference_wrapper")
+ || id_equal (name, "ref_view")));
+ }
+ for (tree fields = TYPE_FIELDS (ctype);
+ fields;
+ fields = DECL_CHAIN (fields))
+ {
+ if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+ continue;
+ tree type = TREE_TYPE (fields);
+ if (!TYPE_REF_P (type))
+ continue;
+ /* OK, the field is a reference member. Do we have a constructor
+ taking its type? */
+ for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
+ {
+ tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+ if (args
+ && same_type_p (TREE_VALUE (args), type)
+ && TREE_CHAIN (args) == void_list_node)
+ return true;
+ }
+ }
+ return false;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13850,7 +13889,36 @@ do_warn_dangling_reference (tree expr)
if (TREE_CODE (arg) == ADDR_EXPR)
arg = TREE_OPERAND (arg, 0);
if (expr_represents_temporary_p (arg))
- return expr;
+ {
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Here we suppose that a member function of such a reference
+ wrapper class returns a reference to a non-temporary object. */
+ if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
+ && !DECL_OVERLOADED_OPERATOR_P (fndecl)
+ && i == 0
+ && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
+ {
+ /* Let's see what the class object was initialized with.
+ ARG is the TARGET_EXPR initializer; it may look like
+ TARGET_EXPR <D.2839, A::foo (decl)>
+ or
+ TARGET_EXPR <D.2839, A::foo (&TARGET_EXPR <...>)>
+ We should only warn for the second case. */
+ while (handled_component_p (arg))
+ arg = TREE_OPERAND (arg, 0);
+ arg = TARGET_EXPR_INITIAL (arg);
+ /* Walk the call chain to the original object and see if
+ it was a temporary. */
+ while (TREE_CODE (arg) == CALL_EXPR)
+ arg = tree_strip_nop_conversions (CALL_EXPR_ARG (arg, 0));
+ if (TREE_CODE (arg) == ADDR_EXPR)
+ arg = TREE_OPERAND (arg, 0);
+ if (!expr_represents_temporary_p (arg))
+ break;
+ }
+ return expr;
+ }
/* Don't warn about member function like:
std::any a(...);
S& s = a.emplace<S>({0}, 0);
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..330de1fd05d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..9ad83f7365e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+ std::reference_wrapper<const X> wrapit() const { return x; }
+ X x;
+};
+
+void
+g (const S& s)
+{
+ const auto& a1 = s.wrapit().get();
+ (void) a1;
+ const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
+ (void) a2;
+}
base-commit: 327d45c57ebd2655a7599df0f01b8b5e2f82eda7
--
2.39.1
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v3] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-01-24 22:49 ` Marek Polacek
@ 2023-02-06 1:25 ` Jason Merrill
2023-02-07 16:46 ` [PATCH v4] " Marek Polacek
0 siblings, 1 reply; 17+ messages in thread
From: Jason Merrill @ 2023-02-06 1:25 UTC (permalink / raw)
To: Marek Polacek; +Cc: GCC Patches
On 1/24/23 17:49, Marek Polacek wrote:
> On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote:
>> On 1/19/23 21:03, Marek Polacek wrote:
>>> On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
>>>> On 1/18/23 20:13, Marek Polacek wrote:
>>>>> On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
>>>>>> On 1/18/23 12:52, Marek Polacek wrote:
>>>>>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>>>>>> some grief. The code in question uses a reference wrapper with a member
>>>>>>> function returning a reference to a subobject of a non-temporary object:
>>>>>>>
>>>>>>> const Plane & meta = fm.planes().inner();
>>>>>>>
>>>>>>> I've tried a few approaches, e.g., checking that the member function's
>>>>>>> return type is the same as the type of the enclosing class (which is
>>>>>>> the case for member functions returning *this), but that then breaks
>>>>>>> Wdangling-reference4.C with std::optional<std::string>.
>>>>>>>
>>>>>>> So I figured that perhaps we want to look at the object we're invoking
>>>>>>> the member function(s) on and see if that is a temporary, as in, don't
>>>>>>> warn about
>>>>>>>
>>>>>>> const Plane & meta = fm.planes().inner();
>>>>>>>
>>>>>>> but do warn about
>>>>>>>
>>>>>>> const Plane & meta = FrameMetadata().planes().inner();
>>>>>>>
>>>>>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>>>>
>>>>>> Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
>>>>>> reference-like type. If planes() returned a class that held data, we would
>>>>>> want to warn.
>>>>>
>>>>> Sure, it's always some kind of tradeoff with warnings :/.
>>>>>> In this case, we might recognize the reference-like class because it has a
>>>>>> reference member and a constructor taking the same reference type.
>>>>>
>>>>> That occurred to me too, but then I found out that std::reference_wrapper
>>>>> actually uses T*, not T&, as you say. But here's a patch to do that
>>>>> (I hope).
>>>>>> That wouldn't help with std::reference_wrapper or std::ref_view because they
>>>>>> have pointer members instead of references, but perhaps loosening the check
>>>>>> to include that case would make sense?
>>>>>
>>>>> Sorry, I don't understand what you mean by loosening the check. I could
>>>>> hardcode std::reference_wrapper and std::ref_view but I don't think that's
>>>>> what you meant.
>>>>
>>>> Indeed that's not what I meant, but as I was saying in our meeting I think
>>>> it's worth doing; the compiler has various tweaks to handle specific
>>>> standard-library classes better.
>>> Okay, done in the patch below. Except that I'm not including a test for
>>> std::ranges::ref_view because I don't really know how that works.
>>>
>>>>> Surely I cannot _not_ warn for any class that contains a T*.
>>>>
>>>> I was thinking if a constructor takes a T& and the class has a T* that would
>>>> be close enough, though this also wouldn't handle the standard library
>>>> classes so the benefit is questionable.
>>>>
>>>>> Here's the patch so that we have some actual code to discuss... Thanks.
>>>>>
>>>>> -- >8 --
>>>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>>>> some grief. The code in question uses a reference wrapper with a member
>>>>> function returning a reference to a subobject of a non-temporary object:
>>>>>
>>>>> const Plane & meta = fm.planes().inner();
>>>>>
>>>>> I've tried a few approaches, e.g., checking that the member function's
>>>>> return type is the same as the type of the enclosing class (which is
>>>>> the case for member functions returning *this), but that then breaks
>>>>> Wdangling-reference4.C with std::optional<std::string>.
>>>>>
>>>>> Perhaps we want to look at the member function's enclosing class
>>>>> to see if it's a reference wrapper class (meaning, has a reference
>>>>> member and a constructor taking the same reference type) and don't
>>>>> warn if so, supposing that the member function returns a reference
>>>>> to a non-temporary object.
>>>>>
>>>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>>>
>>>>> PR c++/107532
>>>>>
>>>>> gcc/cp/ChangeLog:
>>>>>
>>>>> * call.cc (do_warn_dangling_reference): Don't warn when the
>>>>> member function comes from a reference wrapper class.
>>>>
>>>> Let's factor the new code out into e.g. reference_like_class_p
>>>
>>> Done. Thanks,
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> -- >8 --
>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>> some grief. The code in question uses a reference wrapper with a member
>>> function returning a reference to a subobject of a non-temporary object:
>>>
>>> const Plane & meta = fm.planes().inner();
>>>
>>> I've tried a few approaches, e.g., checking that the member function's
>>> return type is the same as the type of the enclosing class (which is
>>> the case for member functions returning *this), but that then breaks
>>> Wdangling-reference4.C with std::optional<std::string>.
>>>
>>> Perhaps we want to look at the member function's enclosing class
>>> to see if it's a reference wrapper class (meaning, has a reference
>>> member and a constructor taking the same reference type, or is
>>> std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
>>> supposing that the member function returns a reference to a non-temporary
>>> object.
>>>
>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>
>>> PR c++/107532
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> * call.cc (reference_like_class_p): New.
>>> (do_warn_dangling_reference): Don't warn when the member function comes
>>> from a reference_like_class_p.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> * g++.dg/warn/Wdangling-reference8.C: New test.
>>> * g++.dg/warn/Wdangling-reference9.C: New test.
>>> ---
>>> gcc/cp/call.cc | 48 ++++++++++++
>>> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
>>> .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
>>> 3 files changed, 146 insertions(+)
>>> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
>>> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>>>
>>> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
>>> index 991730713e6..672722998ee 100644
>>> --- a/gcc/cp/call.cc
>>> +++ b/gcc/cp/call.cc
>>> @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
>>> return true;
>>> }
>>> +/* Return true if a class CTYPE is either std::reference_wrapper or
>>> + std::ref_view, or a reference wrapper class. We consider a class
>>> + a reference wrapper class if it has a reference member and a
>>> + constructor taking the same reference type. */
>>> +
>>> +static bool
>>> +reference_like_class_p (tree ctype)
>>> +{
>>> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
>>> + if (decl_in_std_namespace_p (tdecl))
>>> + {
>>> + tree name = DECL_NAME (tdecl);
>>> + return (name
>>> + && (id_equal (name, "reference_wrapper")
>>> + || id_equal (name, "ref_view")));
>>> + }
>>> + for (tree fields = TYPE_FIELDS (ctype);
>>> + fields;
>>> + fields = DECL_CHAIN (fields))
>>> + {
>>> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
>>> + continue;
>>> + tree type = TREE_TYPE (fields);
>>> + if (!TYPE_REF_P (type))
>>> + continue;
>>> + /* OK, the field is a reference member. Do we have a constructor
>>> + taking its type? */
>>> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
>>> + {
>>> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
>>> + if (args
>>> + && same_type_p (TREE_VALUE (args), type)
>>> + && TREE_CHAIN (args) == void_list_node)
>>> + return true;
>>> + }
>>> + }
>>> + return false;
>>> +}
>>> +
>>> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
>>> that initializes the LHS (and at least one of its arguments represents
>>> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
>>> @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
>>> if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
>>> return NULL_TREE;
>>> + /* An attempt to reduce the number of -Wdangling-reference
>>> + false positives concerning reference wrappers (c++/107532).
>>> + Here we suppose that a member function of such a reference
>>> + wrapper class returns a reference to a non-temporary object. */
>>> + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
>>> + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
>>> + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
>>
>> Ah, in this case I was thinking rather than return we would want to look
>> through to the initializer of the reference wrapper, and warn if that's a
>> temporary, so we can catch the *2 cases in your tests.
>>
>> So, treating ref-like classes as much like references as we can. Some of
>> your v1 patch ought to be useful in implementing this, but only looking
>> through one call at a time, not all of them like that patch.
>
> Maybe this one, then? I still have to loop through the calls though; EXPR in
> do_warn_dangling_reference can be e.g.
>
> Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>)
>
> or
>
> Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>)
>
> and we want to warn only about the latter, but that means that I need to
> look into the nested call 'planes' to see if the initializer was a temporary.
Right, but I was thinking we want to recurse like a few lines above,
rather than loop.
> With this, we warn for the *2 cases too.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> Perhaps we want to look at the member function's enclosing class
> to see if it's a reference wrapper class (meaning, has a reference
> member and a constructor taking the same reference type, or is
> std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
> supposing that the member function returns a reference to a non-temporary
> object.
>
> It's ugly, but better than asking users to add #pragmas into their code.
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (reference_like_class_p): New.
> (do_warn_dangling_reference): Don't warn when the member function comes
> from a reference_like_class_p.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> * g++.dg/warn/Wdangling-reference9.C: New test.
> ---
> gcc/cp/call.cc | 70 ++++++++++++++++-
> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
> .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
> 3 files changed, 167 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 5715a7cd1de..137870670e7 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> return true;
> }
>
> +/* Return true if a class CTYPE is either std::reference_wrapper or
> + std::ref_view, or a reference wrapper class. We consider a class
> + a reference wrapper class if it has a reference member and a
> + constructor taking the same reference type. */
> +
> +static bool
> +reference_like_class_p (tree ctype)
> +{
> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> + if (decl_in_std_namespace_p (tdecl))
> + {
> + tree name = DECL_NAME (tdecl);
> + return (name
> + && (id_equal (name, "reference_wrapper")
> + || id_equal (name, "ref_view")));
> + }
> + for (tree fields = TYPE_FIELDS (ctype);
> + fields;
> + fields = DECL_CHAIN (fields))
> + {
> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> + continue;
> + tree type = TREE_TYPE (fields);
> + if (!TYPE_REF_P (type))
> + continue;
> + /* OK, the field is a reference member. Do we have a constructor
> + taking its type? */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> + {
> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> + if (args
> + && same_type_p (TREE_VALUE (args), type)
> + && TREE_CHAIN (args) == void_list_node)
> + return true;
> + }
> + }
> + return false;
> +}
> +
> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> that initializes the LHS (and at least one of its arguments represents
> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> @@ -13850,7 +13889,36 @@ do_warn_dangling_reference (tree expr)
> if (TREE_CODE (arg) == ADDR_EXPR)
> arg = TREE_OPERAND (arg, 0);
> if (expr_represents_temporary_p (arg))
> - return expr;
> + {
> + /* An attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + Here we suppose that a member function of such a reference
> + wrapper class returns a reference to a non-temporary object. */
> + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> + && i == 0
> + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
> + {
> + /* Let's see what the class object was initialized with.
> + ARG is the TARGET_EXPR initializer; it may look like
> + TARGET_EXPR <D.2839, A::foo (decl)>
> + or
> + TARGET_EXPR <D.2839, A::foo (&TARGET_EXPR <...>)>
> + We should only warn for the second case. */
> + while (handled_component_p (arg))
> + arg = TREE_OPERAND (arg, 0);
> + arg = TARGET_EXPR_INITIAL (arg);
> + /* Walk the call chain to the original object and see if
> + it was a temporary. */
> + while (TREE_CODE (arg) == CALL_EXPR)
> + arg = tree_strip_nop_conversions (CALL_EXPR_ARG (arg, 0));
> + if (TREE_CODE (arg) == ADDR_EXPR)
> + arg = TREE_OPERAND (arg, 0);
> + if (!expr_represents_temporary_p (arg))
> + break;
> + }
> + return expr;
> + }
> /* Don't warn about member function like:
> std::any a(...);
> S& s = a.emplace<S>({0}, 0);
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..330de1fd05d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner();
> + (void) d8;
> +}
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> new file mode 100644
> index 00000000000..9ad83f7365e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> @@ -0,0 +1,21 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <functional>
> +
> +struct X { int n; };
> +
> +struct S {
> + std::reference_wrapper<const X> wrapit() const { return x; }
> + X x;
> +};
> +
> +void
> +g (const S& s)
> +{
> + const auto& a1 = s.wrapit().get();
> + (void) a1;
> + const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
> + (void) a2;
> +}
>
> base-commit: 327d45c57ebd2655a7599df0f01b8b5e2f82eda7
^ permalink raw reply [flat|nested] 17+ messages in thread
* [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-02-06 1:25 ` Jason Merrill
@ 2023-02-07 16:46 ` Marek Polacek
2023-03-01 20:34 ` Marek Polacek
2023-03-01 21:53 ` Jason Merrill
0 siblings, 2 replies; 17+ messages in thread
From: Marek Polacek @ 2023-02-07 16:46 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Sun, Feb 05, 2023 at 05:25:25PM -0800, Jason Merrill wrote:
> On 1/24/23 17:49, Marek Polacek wrote:
> > On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote:
> > > On 1/19/23 21:03, Marek Polacek wrote:
> > > > On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
> > > > > On 1/18/23 20:13, Marek Polacek wrote:
> > > > > > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> > > > > > > On 1/18/23 12:52, Marek Polacek wrote:
> > > > > > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > > > > > some grief. The code in question uses a reference wrapper with a member
> > > > > > > > function returning a reference to a subobject of a non-temporary object:
> > > > > > > >
> > > > > > > > const Plane & meta = fm.planes().inner();
> > > > > > > >
> > > > > > > > I've tried a few approaches, e.g., checking that the member function's
> > > > > > > > return type is the same as the type of the enclosing class (which is
> > > > > > > > the case for member functions returning *this), but that then breaks
> > > > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > > > >
> > > > > > > > So I figured that perhaps we want to look at the object we're invoking
> > > > > > > > the member function(s) on and see if that is a temporary, as in, don't
> > > > > > > > warn about
> > > > > > > >
> > > > > > > > const Plane & meta = fm.planes().inner();
> > > > > > > >
> > > > > > > > but do warn about
> > > > > > > >
> > > > > > > > const Plane & meta = FrameMetadata().planes().inner();
> > > > > > > >
> > > > > > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > > > >
> > > > > > > Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
> > > > > > > reference-like type. If planes() returned a class that held data, we would
> > > > > > > want to warn.
> > > > > >
> > > > > > Sure, it's always some kind of tradeoff with warnings :/.
> > > > > > > In this case, we might recognize the reference-like class because it has a
> > > > > > > reference member and a constructor taking the same reference type.
> > > > > >
> > > > > > That occurred to me too, but then I found out that std::reference_wrapper
> > > > > > actually uses T*, not T&, as you say. But here's a patch to do that
> > > > > > (I hope).
> > > > > > > That wouldn't help with std::reference_wrapper or std::ref_view because they
> > > > > > > have pointer members instead of references, but perhaps loosening the check
> > > > > > > to include that case would make sense?
> > > > > >
> > > > > > Sorry, I don't understand what you mean by loosening the check. I could
> > > > > > hardcode std::reference_wrapper and std::ref_view but I don't think that's
> > > > > > what you meant.
> > > > >
> > > > > Indeed that's not what I meant, but as I was saying in our meeting I think
> > > > > it's worth doing; the compiler has various tweaks to handle specific
> > > > > standard-library classes better.
> > > > Okay, done in the patch below. Except that I'm not including a test for
> > > > std::ranges::ref_view because I don't really know how that works.
> > > >
> > > > > > Surely I cannot _not_ warn for any class that contains a T*.
> > > > >
> > > > > I was thinking if a constructor takes a T& and the class has a T* that would
> > > > > be close enough, though this also wouldn't handle the standard library
> > > > > classes so the benefit is questionable.
> > > > >
> > > > > > Here's the patch so that we have some actual code to discuss... Thanks.
> > > > > >
> > > > > > -- >8 --
> > > > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > > > some grief. The code in question uses a reference wrapper with a member
> > > > > > function returning a reference to a subobject of a non-temporary object:
> > > > > >
> > > > > > const Plane & meta = fm.planes().inner();
> > > > > >
> > > > > > I've tried a few approaches, e.g., checking that the member function's
> > > > > > return type is the same as the type of the enclosing class (which is
> > > > > > the case for member functions returning *this), but that then breaks
> > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > >
> > > > > > Perhaps we want to look at the member function's enclosing class
> > > > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > > > member and a constructor taking the same reference type) and don't
> > > > > > warn if so, supposing that the member function returns a reference
> > > > > > to a non-temporary object.
> > > > > >
> > > > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > > >
> > > > > > PR c++/107532
> > > > > >
> > > > > > gcc/cp/ChangeLog:
> > > > > >
> > > > > > * call.cc (do_warn_dangling_reference): Don't warn when the
> > > > > > member function comes from a reference wrapper class.
> > > > >
> > > > > Let's factor the new code out into e.g. reference_like_class_p
> > > >
> > > > Done. Thanks,
> > > >
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > >
> > > > -- >8 --
> > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > some grief. The code in question uses a reference wrapper with a member
> > > > function returning a reference to a subobject of a non-temporary object:
> > > >
> > > > const Plane & meta = fm.planes().inner();
> > > >
> > > > I've tried a few approaches, e.g., checking that the member function's
> > > > return type is the same as the type of the enclosing class (which is
> > > > the case for member functions returning *this), but that then breaks
> > > > Wdangling-reference4.C with std::optional<std::string>.
> > > >
> > > > Perhaps we want to look at the member function's enclosing class
> > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > member and a constructor taking the same reference type, or is
> > > > std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
> > > > supposing that the member function returns a reference to a non-temporary
> > > > object.
> > > >
> > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > >
> > > > PR c++/107532
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > > * call.cc (reference_like_class_p): New.
> > > > (do_warn_dangling_reference): Don't warn when the member function comes
> > > > from a reference_like_class_p.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > * g++.dg/warn/Wdangling-reference8.C: New test.
> > > > * g++.dg/warn/Wdangling-reference9.C: New test.
> > > > ---
> > > > gcc/cp/call.cc | 48 ++++++++++++
> > > > .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
> > > > .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
> > > > 3 files changed, 146 insertions(+)
> > > > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> > > > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> > > >
> > > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > > index 991730713e6..672722998ee 100644
> > > > --- a/gcc/cp/call.cc
> > > > +++ b/gcc/cp/call.cc
> > > > @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> > > > return true;
> > > > }
> > > > +/* Return true if a class CTYPE is either std::reference_wrapper or
> > > > + std::ref_view, or a reference wrapper class. We consider a class
> > > > + a reference wrapper class if it has a reference member and a
> > > > + constructor taking the same reference type. */
> > > > +
> > > > +static bool
> > > > +reference_like_class_p (tree ctype)
> > > > +{
> > > > + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> > > > + if (decl_in_std_namespace_p (tdecl))
> > > > + {
> > > > + tree name = DECL_NAME (tdecl);
> > > > + return (name
> > > > + && (id_equal (name, "reference_wrapper")
> > > > + || id_equal (name, "ref_view")));
> > > > + }
> > > > + for (tree fields = TYPE_FIELDS (ctype);
> > > > + fields;
> > > > + fields = DECL_CHAIN (fields))
> > > > + {
> > > > + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> > > > + continue;
> > > > + tree type = TREE_TYPE (fields);
> > > > + if (!TYPE_REF_P (type))
> > > > + continue;
> > > > + /* OK, the field is a reference member. Do we have a constructor
> > > > + taking its type? */
> > > > + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> > > > + {
> > > > + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> > > > + if (args
> > > > + && same_type_p (TREE_VALUE (args), type)
> > > > + && TREE_CHAIN (args) == void_list_node)
> > > > + return true;
> > > > + }
> > > > + }
> > > > + return false;
> > > > +}
> > > > +
> > > > /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> > > > that initializes the LHS (and at least one of its arguments represents
> > > > a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> > > > @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
> > > > if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> > > > return NULL_TREE;
> > > > + /* An attempt to reduce the number of -Wdangling-reference
> > > > + false positives concerning reference wrappers (c++/107532).
> > > > + Here we suppose that a member function of such a reference
> > > > + wrapper class returns a reference to a non-temporary object. */
> > > > + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > > > + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> > > > + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
> > >
> > > Ah, in this case I was thinking rather than return we would want to look
> > > through to the initializer of the reference wrapper, and warn if that's a
> > > temporary, so we can catch the *2 cases in your tests.
> > >
> > > So, treating ref-like classes as much like references as we can. Some of
> > > your v1 patch ought to be useful in implementing this, but only looking
> > > through one call at a time, not all of them like that patch.
> >
> > Maybe this one, then? I still have to loop through the calls though; EXPR in
> > do_warn_dangling_reference can be e.g.
> >
> > Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>)
> >
> > or
> >
> > Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>)
> >
> > and we want to warn only about the latter, but that means that I need to
> > look into the nested call 'planes' to see if the initializer was a temporary.
>
> Right, but I was thinking we want to recurse like a few lines above, rather
> than loop.
Ah yes, I can do that if I introduce a parameter that tells us
if we're processing an argument or not. I think I'm finally
more or less satisfied with the patch, thanks.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
This patch adjusts do_warn_dangling_reference so that we look through
reference wrapper classes (meaning, has a reference member and a
constructor taking the same reference type, or is std::reference_wrapper
or std::ranges::ref_view) and don't warn for them, supposing that the
member function returns a reference to a non-temporary object.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Add new bool parameter. See through
reference_like_class_p.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
* g++.dg/warn/Wdangling-reference9.C: New test.
---
gcc/cp/call.cc | 97 +++++++++++++++----
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 ++++
3 files changed, 178 insertions(+), 17 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f7c5d9da94b..2a8edc2e7e2 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
return true;
}
+/* Return true if a class CTYPE is either std::reference_wrapper or
+ std::ref_view, or a reference wrapper class. We consider a class
+ a reference wrapper class if it has a reference member and a
+ constructor taking the same reference type. */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+ if (decl_in_std_namespace_p (tdecl))
+ {
+ tree name = DECL_NAME (tdecl);
+ return (name
+ && (id_equal (name, "reference_wrapper")
+ || id_equal (name, "ref_view")));
+ }
+ for (tree fields = TYPE_FIELDS (ctype);
+ fields;
+ fields = DECL_CHAIN (fields))
+ {
+ if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+ continue;
+ tree type = TREE_TYPE (fields);
+ if (!TYPE_REF_P (type))
+ continue;
+ /* OK, the field is a reference member. Do we have a constructor
+ taking its type? */
+ for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
+ {
+ tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+ if (args
+ && same_type_p (TREE_VALUE (args), type)
+ && TREE_CHAIN (args) == void_list_node)
+ return true;
+ }
+ }
+ return false;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
const int& y = (f(1), 42); // NULL_TREE
const int& z = f(f(1)); // f(f(1))
- EXPR is the initializer. */
+ EXPR is the initializer. If ARG_P is true, we're processing an argument
+ to a function; the point is to distinguish between, for example,
+
+ Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
+
+ where we shouldn't warn, and
+
+ Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
+
+ where we should warn (Ref is a reference_like_class_p so we see through
+ it. */
static tree
-do_warn_dangling_reference (tree expr)
+do_warn_dangling_reference (tree expr, bool arg_p)
{
STRIP_NOPS (expr);
+ if (TREE_CODE (expr) == ADDR_EXPR)
+ expr = TREE_OPERAND (expr, 0);
+
+ if (arg_p && expr_represents_temporary_p (expr))
+ {
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Here we suppose that a member function of such a reference
+ wrapper class returns a reference to a non-temporary object. */
+ tree e = expr;
+ while (handled_component_p (e))
+ e = TREE_OPERAND (e, 0);
+ e = TREE_TYPE (e);
+ if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
+ return expr;
+ }
+
switch (TREE_CODE (expr))
{
case CALL_EXPR:
@@ -13829,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
std::pair<const int&, const int&> v = std::minmax(1, 2);
which also creates a dangling reference, because std::minmax
returns std::pair<const T&, const T&>(b, a). */
- if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
+ if (!arg_p
+ && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
return NULL_TREE;
/* Here we're looking to see if any of the arguments is a temporary
@@ -13842,14 +13909,10 @@ do_warn_dangling_reference (tree expr)
if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
&& !TYPE_REF_P (TREE_TYPE (arg)))
continue;
- /* It could also be another call taking a temporary and returning
- it and initializing this reference parameter. */
- if (do_warn_dangling_reference (arg))
- return expr;
- STRIP_NOPS (arg);
- if (TREE_CODE (arg) == ADDR_EXPR)
- arg = TREE_OPERAND (arg, 0);
- if (expr_represents_temporary_p (arg))
+ /* Recurse to see if the argument is a temporary. It could also
+ be another call taking a temporary and returning it and
+ initializing this reference parameter. */
+ if (do_warn_dangling_reference (arg, /*arg_p=*/true))
return expr;
/* Don't warn about member function like:
std::any a(...);
@@ -13866,15 +13929,15 @@ do_warn_dangling_reference (tree expr)
return NULL_TREE;
}
case COMPOUND_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
case COND_EXPR:
- if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
+ if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
return t;
- return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
case PAREN_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
case TARGET_EXPR:
- return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
+ return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
default:
return NULL_TREE;
}
@@ -13917,7 +13980,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
= make_temp_override (global_dc->dc_warn_system_headers,
(!in_system_header_at (DECL_SOURCE_LOCATION (decl))
|| global_dc->dc_warn_system_headers));
- if (tree call = do_warn_dangling_reference (init))
+ if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
{
auto_diagnostic_group d;
if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..330de1fd05d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..9ad83f7365e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+ std::reference_wrapper<const X> wrapit() const { return x; }
+ X x;
+};
+
+void
+g (const S& s)
+{
+ const auto& a1 = s.wrapit().get();
+ (void) a1;
+ const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
+ (void) a2;
+}
base-commit: f661c0bb6371f355966a67b5ce71398e80792948
--
2.39.1
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-02-07 16:46 ` [PATCH v4] " Marek Polacek
@ 2023-03-01 20:34 ` Marek Polacek
2023-03-01 21:53 ` Jason Merrill
1 sibling, 0 replies; 17+ messages in thread
From: Marek Polacek @ 2023-03-01 20:34 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
Ping.
On Tue, Feb 07, 2023 at 11:46:10AM -0500, Marek Polacek wrote:
> On Sun, Feb 05, 2023 at 05:25:25PM -0800, Jason Merrill wrote:
> > On 1/24/23 17:49, Marek Polacek wrote:
> > > On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote:
> > > > On 1/19/23 21:03, Marek Polacek wrote:
> > > > > On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
> > > > > > On 1/18/23 20:13, Marek Polacek wrote:
> > > > > > > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
> > > > > > > > On 1/18/23 12:52, Marek Polacek wrote:
> > > > > > > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > > > > > > some grief. The code in question uses a reference wrapper with a member
> > > > > > > > > function returning a reference to a subobject of a non-temporary object:
> > > > > > > > >
> > > > > > > > > const Plane & meta = fm.planes().inner();
> > > > > > > > >
> > > > > > > > > I've tried a few approaches, e.g., checking that the member function's
> > > > > > > > > return type is the same as the type of the enclosing class (which is
> > > > > > > > > the case for member functions returning *this), but that then breaks
> > > > > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > > > > >
> > > > > > > > > So I figured that perhaps we want to look at the object we're invoking
> > > > > > > > > the member function(s) on and see if that is a temporary, as in, don't
> > > > > > > > > warn about
> > > > > > > > >
> > > > > > > > > const Plane & meta = fm.planes().inner();
> > > > > > > > >
> > > > > > > > > but do warn about
> > > > > > > > >
> > > > > > > > > const Plane & meta = FrameMetadata().planes().inner();
> > > > > > > > >
> > > > > > > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > > > > >
> > > > > > > > Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
> > > > > > > > reference-like type. If planes() returned a class that held data, we would
> > > > > > > > want to warn.
> > > > > > >
> > > > > > > Sure, it's always some kind of tradeoff with warnings :/.
> > > > > > > > In this case, we might recognize the reference-like class because it has a
> > > > > > > > reference member and a constructor taking the same reference type.
> > > > > > >
> > > > > > > That occurred to me too, but then I found out that std::reference_wrapper
> > > > > > > actually uses T*, not T&, as you say. But here's a patch to do that
> > > > > > > (I hope).
> > > > > > > > That wouldn't help with std::reference_wrapper or std::ref_view because they
> > > > > > > > have pointer members instead of references, but perhaps loosening the check
> > > > > > > > to include that case would make sense?
> > > > > > >
> > > > > > > Sorry, I don't understand what you mean by loosening the check. I could
> > > > > > > hardcode std::reference_wrapper and std::ref_view but I don't think that's
> > > > > > > what you meant.
> > > > > >
> > > > > > Indeed that's not what I meant, but as I was saying in our meeting I think
> > > > > > it's worth doing; the compiler has various tweaks to handle specific
> > > > > > standard-library classes better.
> > > > > Okay, done in the patch below. Except that I'm not including a test for
> > > > > std::ranges::ref_view because I don't really know how that works.
> > > > >
> > > > > > > Surely I cannot _not_ warn for any class that contains a T*.
> > > > > >
> > > > > > I was thinking if a constructor takes a T& and the class has a T* that would
> > > > > > be close enough, though this also wouldn't handle the standard library
> > > > > > classes so the benefit is questionable.
> > > > > >
> > > > > > > Here's the patch so that we have some actual code to discuss... Thanks.
> > > > > > >
> > > > > > > -- >8 --
> > > > > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > > > > some grief. The code in question uses a reference wrapper with a member
> > > > > > > function returning a reference to a subobject of a non-temporary object:
> > > > > > >
> > > > > > > const Plane & meta = fm.planes().inner();
> > > > > > >
> > > > > > > I've tried a few approaches, e.g., checking that the member function's
> > > > > > > return type is the same as the type of the enclosing class (which is
> > > > > > > the case for member functions returning *this), but that then breaks
> > > > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > > > >
> > > > > > > Perhaps we want to look at the member function's enclosing class
> > > > > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > > > > member and a constructor taking the same reference type) and don't
> > > > > > > warn if so, supposing that the member function returns a reference
> > > > > > > to a non-temporary object.
> > > > > > >
> > > > > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > > > >
> > > > > > > PR c++/107532
> > > > > > >
> > > > > > > gcc/cp/ChangeLog:
> > > > > > >
> > > > > > > * call.cc (do_warn_dangling_reference): Don't warn when the
> > > > > > > member function comes from a reference wrapper class.
> > > > > >
> > > > > > Let's factor the new code out into e.g. reference_like_class_p
> > > > >
> > > > > Done. Thanks,
> > > > >
> > > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > > >
> > > > > -- >8 --
> > > > > Here, -Wdangling-reference triggers where it probably shouldn't, causing
> > > > > some grief. The code in question uses a reference wrapper with a member
> > > > > function returning a reference to a subobject of a non-temporary object:
> > > > >
> > > > > const Plane & meta = fm.planes().inner();
> > > > >
> > > > > I've tried a few approaches, e.g., checking that the member function's
> > > > > return type is the same as the type of the enclosing class (which is
> > > > > the case for member functions returning *this), but that then breaks
> > > > > Wdangling-reference4.C with std::optional<std::string>.
> > > > >
> > > > > Perhaps we want to look at the member function's enclosing class
> > > > > to see if it's a reference wrapper class (meaning, has a reference
> > > > > member and a constructor taking the same reference type, or is
> > > > > std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
> > > > > supposing that the member function returns a reference to a non-temporary
> > > > > object.
> > > > >
> > > > > It's ugly, but better than asking users to add #pragmas into their code.
> > > > >
> > > > > PR c++/107532
> > > > >
> > > > > gcc/cp/ChangeLog:
> > > > >
> > > > > * call.cc (reference_like_class_p): New.
> > > > > (do_warn_dangling_reference): Don't warn when the member function comes
> > > > > from a reference_like_class_p.
> > > > >
> > > > > gcc/testsuite/ChangeLog:
> > > > >
> > > > > * g++.dg/warn/Wdangling-reference8.C: New test.
> > > > > * g++.dg/warn/Wdangling-reference9.C: New test.
> > > > > ---
> > > > > gcc/cp/call.cc | 48 ++++++++++++
> > > > > .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
> > > > > .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
> > > > > 3 files changed, 146 insertions(+)
> > > > > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> > > > > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> > > > >
> > > > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > > > index 991730713e6..672722998ee 100644
> > > > > --- a/gcc/cp/call.cc
> > > > > +++ b/gcc/cp/call.cc
> > > > > @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> > > > > return true;
> > > > > }
> > > > > +/* Return true if a class CTYPE is either std::reference_wrapper or
> > > > > + std::ref_view, or a reference wrapper class. We consider a class
> > > > > + a reference wrapper class if it has a reference member and a
> > > > > + constructor taking the same reference type. */
> > > > > +
> > > > > +static bool
> > > > > +reference_like_class_p (tree ctype)
> > > > > +{
> > > > > + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> > > > > + if (decl_in_std_namespace_p (tdecl))
> > > > > + {
> > > > > + tree name = DECL_NAME (tdecl);
> > > > > + return (name
> > > > > + && (id_equal (name, "reference_wrapper")
> > > > > + || id_equal (name, "ref_view")));
> > > > > + }
> > > > > + for (tree fields = TYPE_FIELDS (ctype);
> > > > > + fields;
> > > > > + fields = DECL_CHAIN (fields))
> > > > > + {
> > > > > + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> > > > > + continue;
> > > > > + tree type = TREE_TYPE (fields);
> > > > > + if (!TYPE_REF_P (type))
> > > > > + continue;
> > > > > + /* OK, the field is a reference member. Do we have a constructor
> > > > > + taking its type? */
> > > > > + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> > > > > + {
> > > > > + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> > > > > + if (args
> > > > > + && same_type_p (TREE_VALUE (args), type)
> > > > > + && TREE_CHAIN (args) == void_list_node)
> > > > > + return true;
> > > > > + }
> > > > > + }
> > > > > + return false;
> > > > > +}
> > > > > +
> > > > > /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> > > > > that initializes the LHS (and at least one of its arguments represents
> > > > > a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> > > > > @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
> > > > > if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> > > > > return NULL_TREE;
> > > > > + /* An attempt to reduce the number of -Wdangling-reference
> > > > > + false positives concerning reference wrappers (c++/107532).
> > > > > + Here we suppose that a member function of such a reference
> > > > > + wrapper class returns a reference to a non-temporary object. */
> > > > > + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > > > > + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
> > > > > + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
> > > >
> > > > Ah, in this case I was thinking rather than return we would want to look
> > > > through to the initializer of the reference wrapper, and warn if that's a
> > > > temporary, so we can catch the *2 cases in your tests.
> > > >
> > > > So, treating ref-like classes as much like references as we can. Some of
> > > > your v1 patch ought to be useful in implementing this, but only looking
> > > > through one call at a time, not all of them like that patch.
> > >
> > > Maybe this one, then? I still have to loop through the calls though; EXPR in
> > > do_warn_dangling_reference can be e.g.
> > >
> > > Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>)
> > >
> > > or
> > >
> > > Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>)
> > >
> > > and we want to warn only about the latter, but that means that I need to
> > > look into the nested call 'planes' to see if the initializer was a temporary.
> >
> > Right, but I was thinking we want to recurse like a few lines above, rather
> > than loop.
>
> Ah yes, I can do that if I introduce a parameter that tells us
> if we're processing an argument or not. I think I'm finally
> more or less satisfied with the patch, thanks.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> This patch adjusts do_warn_dangling_reference so that we look through
> reference wrapper classes (meaning, has a reference member and a
> constructor taking the same reference type, or is std::reference_wrapper
> or std::ranges::ref_view) and don't warn for them, supposing that the
> member function returns a reference to a non-temporary object.
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (reference_like_class_p): New.
> (do_warn_dangling_reference): Add new bool parameter. See through
> reference_like_class_p.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> * g++.dg/warn/Wdangling-reference9.C: New test.
> ---
> gcc/cp/call.cc | 97 +++++++++++++++----
> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++
> .../g++.dg/warn/Wdangling-reference9.C | 21 ++++
> 3 files changed, 178 insertions(+), 17 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index f7c5d9da94b..2a8edc2e7e2 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> return true;
> }
>
> +/* Return true if a class CTYPE is either std::reference_wrapper or
> + std::ref_view, or a reference wrapper class. We consider a class
> + a reference wrapper class if it has a reference member and a
> + constructor taking the same reference type. */
> +
> +static bool
> +reference_like_class_p (tree ctype)
> +{
> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> + if (decl_in_std_namespace_p (tdecl))
> + {
> + tree name = DECL_NAME (tdecl);
> + return (name
> + && (id_equal (name, "reference_wrapper")
> + || id_equal (name, "ref_view")));
> + }
> + for (tree fields = TYPE_FIELDS (ctype);
> + fields;
> + fields = DECL_CHAIN (fields))
> + {
> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> + continue;
> + tree type = TREE_TYPE (fields);
> + if (!TYPE_REF_P (type))
> + continue;
> + /* OK, the field is a reference member. Do we have a constructor
> + taking its type? */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> + {
> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> + if (args
> + && same_type_p (TREE_VALUE (args), type)
> + && TREE_CHAIN (args) == void_list_node)
> + return true;
> + }
> + }
> + return false;
> +}
> +
> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> that initializes the LHS (and at least one of its arguments represents
> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
> const int& y = (f(1), 42); // NULL_TREE
> const int& z = f(f(1)); // f(f(1))
>
> - EXPR is the initializer. */
> + EXPR is the initializer. If ARG_P is true, we're processing an argument
> + to a function; the point is to distinguish between, for example,
> +
> + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
> +
> + where we shouldn't warn, and
> +
> + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
> +
> + where we should warn (Ref is a reference_like_class_p so we see through
> + it. */
>
> static tree
> -do_warn_dangling_reference (tree expr)
> +do_warn_dangling_reference (tree expr, bool arg_p)
> {
> STRIP_NOPS (expr);
> + if (TREE_CODE (expr) == ADDR_EXPR)
> + expr = TREE_OPERAND (expr, 0);
> +
> + if (arg_p && expr_represents_temporary_p (expr))
> + {
> + /* An attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + Here we suppose that a member function of such a reference
> + wrapper class returns a reference to a non-temporary object. */
> + tree e = expr;
> + while (handled_component_p (e))
> + e = TREE_OPERAND (e, 0);
> + e = TREE_TYPE (e);
> + if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
> + return expr;
> + }
> +
> switch (TREE_CODE (expr))
> {
> case CALL_EXPR:
> @@ -13829,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
> std::pair<const int&, const int&> v = std::minmax(1, 2);
> which also creates a dangling reference, because std::minmax
> returns std::pair<const T&, const T&>(b, a). */
> - if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> + if (!arg_p
> + && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
> return NULL_TREE;
>
> /* Here we're looking to see if any of the arguments is a temporary
> @@ -13842,14 +13909,10 @@ do_warn_dangling_reference (tree expr)
> if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> && !TYPE_REF_P (TREE_TYPE (arg)))
> continue;
> - /* It could also be another call taking a temporary and returning
> - it and initializing this reference parameter. */
> - if (do_warn_dangling_reference (arg))
> - return expr;
> - STRIP_NOPS (arg);
> - if (TREE_CODE (arg) == ADDR_EXPR)
> - arg = TREE_OPERAND (arg, 0);
> - if (expr_represents_temporary_p (arg))
> + /* Recurse to see if the argument is a temporary. It could also
> + be another call taking a temporary and returning it and
> + initializing this reference parameter. */
> + if (do_warn_dangling_reference (arg, /*arg_p=*/true))
> return expr;
> /* Don't warn about member function like:
> std::any a(...);
> @@ -13866,15 +13929,15 @@ do_warn_dangling_reference (tree expr)
> return NULL_TREE;
> }
> case COMPOUND_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
> case COND_EXPR:
> - if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
> + if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
> return t;
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
> case PAREN_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
> case TARGET_EXPR:
> - return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
> + return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
> default:
> return NULL_TREE;
> }
> @@ -13917,7 +13980,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
> = make_temp_override (global_dc->dc_warn_system_headers,
> (!in_system_header_at (DECL_SOURCE_LOCATION (decl))
> || global_dc->dc_warn_system_headers));
> - if (tree call = do_warn_dangling_reference (init))
> + if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
> {
> auto_diagnostic_group d;
> if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..330de1fd05d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner();
> + (void) d8;
> +}
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> new file mode 100644
> index 00000000000..9ad83f7365e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> @@ -0,0 +1,21 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <functional>
> +
> +struct X { int n; };
> +
> +struct S {
> + std::reference_wrapper<const X> wrapit() const { return x; }
> + X x;
> +};
> +
> +void
> +g (const S& s)
> +{
> + const auto& a1 = s.wrapit().get();
> + (void) a1;
> + const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
> + (void) a2;
> +}
>
> base-commit: f661c0bb6371f355966a67b5ce71398e80792948
> --
> 2.39.1
>
Marek
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-02-07 16:46 ` [PATCH v4] " Marek Polacek
2023-03-01 20:34 ` Marek Polacek
@ 2023-03-01 21:53 ` Jason Merrill
2023-03-02 21:24 ` Marek Polacek
1 sibling, 1 reply; 17+ messages in thread
From: Jason Merrill @ 2023-03-01 21:53 UTC (permalink / raw)
To: Marek Polacek; +Cc: GCC Patches
On 2/7/23 11:46, Marek Polacek wrote:
> On Sun, Feb 05, 2023 at 05:25:25PM -0800, Jason Merrill wrote:
>> On 1/24/23 17:49, Marek Polacek wrote:
>>> On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote:
>>>> On 1/19/23 21:03, Marek Polacek wrote:
>>>>> On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote:
>>>>>> On 1/18/23 20:13, Marek Polacek wrote:
>>>>>>> On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote:
>>>>>>>> On 1/18/23 12:52, Marek Polacek wrote:
>>>>>>>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>>>>>>>> some grief. The code in question uses a reference wrapper with a member
>>>>>>>>> function returning a reference to a subobject of a non-temporary object:
>>>>>>>>>
>>>>>>>>> const Plane & meta = fm.planes().inner();
>>>>>>>>>
>>>>>>>>> I've tried a few approaches, e.g., checking that the member function's
>>>>>>>>> return type is the same as the type of the enclosing class (which is
>>>>>>>>> the case for member functions returning *this), but that then breaks
>>>>>>>>> Wdangling-reference4.C with std::optional<std::string>.
>>>>>>>>>
>>>>>>>>> So I figured that perhaps we want to look at the object we're invoking
>>>>>>>>> the member function(s) on and see if that is a temporary, as in, don't
>>>>>>>>> warn about
>>>>>>>>>
>>>>>>>>> const Plane & meta = fm.planes().inner();
>>>>>>>>>
>>>>>>>>> but do warn about
>>>>>>>>>
>>>>>>>>> const Plane & meta = FrameMetadata().planes().inner();
>>>>>>>>>
>>>>>>>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>>>>>>
>>>>>>>> Hmm, that doesn't seem right; the former is only OK because Ref is in fact a
>>>>>>>> reference-like type. If planes() returned a class that held data, we would
>>>>>>>> want to warn.
>>>>>>>
>>>>>>> Sure, it's always some kind of tradeoff with warnings :/.
>>>>>>>> In this case, we might recognize the reference-like class because it has a
>>>>>>>> reference member and a constructor taking the same reference type.
>>>>>>>
>>>>>>> That occurred to me too, but then I found out that std::reference_wrapper
>>>>>>> actually uses T*, not T&, as you say. But here's a patch to do that
>>>>>>> (I hope).
>>>>>>>> That wouldn't help with std::reference_wrapper or std::ref_view because they
>>>>>>>> have pointer members instead of references, but perhaps loosening the check
>>>>>>>> to include that case would make sense?
>>>>>>>
>>>>>>> Sorry, I don't understand what you mean by loosening the check. I could
>>>>>>> hardcode std::reference_wrapper and std::ref_view but I don't think that's
>>>>>>> what you meant.
>>>>>>
>>>>>> Indeed that's not what I meant, but as I was saying in our meeting I think
>>>>>> it's worth doing; the compiler has various tweaks to handle specific
>>>>>> standard-library classes better.
>>>>> Okay, done in the patch below. Except that I'm not including a test for
>>>>> std::ranges::ref_view because I don't really know how that works.
>>>>>
>>>>>>> Surely I cannot _not_ warn for any class that contains a T*.
>>>>>>
>>>>>> I was thinking if a constructor takes a T& and the class has a T* that would
>>>>>> be close enough, though this also wouldn't handle the standard library
>>>>>> classes so the benefit is questionable.
>>>>>>
>>>>>>> Here's the patch so that we have some actual code to discuss... Thanks.
>>>>>>>
>>>>>>> -- >8 --
>>>>>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>>>>>> some grief. The code in question uses a reference wrapper with a member
>>>>>>> function returning a reference to a subobject of a non-temporary object:
>>>>>>>
>>>>>>> const Plane & meta = fm.planes().inner();
>>>>>>>
>>>>>>> I've tried a few approaches, e.g., checking that the member function's
>>>>>>> return type is the same as the type of the enclosing class (which is
>>>>>>> the case for member functions returning *this), but that then breaks
>>>>>>> Wdangling-reference4.C with std::optional<std::string>.
>>>>>>>
>>>>>>> Perhaps we want to look at the member function's enclosing class
>>>>>>> to see if it's a reference wrapper class (meaning, has a reference
>>>>>>> member and a constructor taking the same reference type) and don't
>>>>>>> warn if so, supposing that the member function returns a reference
>>>>>>> to a non-temporary object.
>>>>>>>
>>>>>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>>>>>
>>>>>>> PR c++/107532
>>>>>>>
>>>>>>> gcc/cp/ChangeLog:
>>>>>>>
>>>>>>> * call.cc (do_warn_dangling_reference): Don't warn when the
>>>>>>> member function comes from a reference wrapper class.
>>>>>>
>>>>>> Let's factor the new code out into e.g. reference_like_class_p
>>>>>
>>>>> Done. Thanks,
>>>>>
>>>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>>>
>>>>> -- >8 --
>>>>> Here, -Wdangling-reference triggers where it probably shouldn't, causing
>>>>> some grief. The code in question uses a reference wrapper with a member
>>>>> function returning a reference to a subobject of a non-temporary object:
>>>>>
>>>>> const Plane & meta = fm.planes().inner();
>>>>>
>>>>> I've tried a few approaches, e.g., checking that the member function's
>>>>> return type is the same as the type of the enclosing class (which is
>>>>> the case for member functions returning *this), but that then breaks
>>>>> Wdangling-reference4.C with std::optional<std::string>.
>>>>>
>>>>> Perhaps we want to look at the member function's enclosing class
>>>>> to see if it's a reference wrapper class (meaning, has a reference
>>>>> member and a constructor taking the same reference type, or is
>>>>> std::reference_wrapper or std::ranges::ref_view) and don't warn if so,
>>>>> supposing that the member function returns a reference to a non-temporary
>>>>> object.
>>>>>
>>>>> It's ugly, but better than asking users to add #pragmas into their code.
>>>>>
>>>>> PR c++/107532
>>>>>
>>>>> gcc/cp/ChangeLog:
>>>>>
>>>>> * call.cc (reference_like_class_p): New.
>>>>> (do_warn_dangling_reference): Don't warn when the member function comes
>>>>> from a reference_like_class_p.
>>>>>
>>>>> gcc/testsuite/ChangeLog:
>>>>>
>>>>> * g++.dg/warn/Wdangling-reference8.C: New test.
>>>>> * g++.dg/warn/Wdangling-reference9.C: New test.
>>>>> ---
>>>>> gcc/cp/call.cc | 48 ++++++++++++
>>>>> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++
>>>>> .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
>>>>> 3 files changed, 146 insertions(+)
>>>>> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
>>>>> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>>>>>
>>>>> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
>>>>> index 991730713e6..672722998ee 100644
>>>>> --- a/gcc/cp/call.cc
>>>>> +++ b/gcc/cp/call.cc
>>>>> @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
>>>>> return true;
>>>>> }
>>>>> +/* Return true if a class CTYPE is either std::reference_wrapper or
>>>>> + std::ref_view, or a reference wrapper class. We consider a class
>>>>> + a reference wrapper class if it has a reference member and a
>>>>> + constructor taking the same reference type. */
>>>>> +
>>>>> +static bool
>>>>> +reference_like_class_p (tree ctype)
>>>>> +{
>>>>> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
>>>>> + if (decl_in_std_namespace_p (tdecl))
>>>>> + {
>>>>> + tree name = DECL_NAME (tdecl);
>>>>> + return (name
>>>>> + && (id_equal (name, "reference_wrapper")
>>>>> + || id_equal (name, "ref_view")));
>>>>> + }
>>>>> + for (tree fields = TYPE_FIELDS (ctype);
>>>>> + fields;
>>>>> + fields = DECL_CHAIN (fields))
>>>>> + {
>>>>> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
>>>>> + continue;
>>>>> + tree type = TREE_TYPE (fields);
>>>>> + if (!TYPE_REF_P (type))
>>>>> + continue;
>>>>> + /* OK, the field is a reference member. Do we have a constructor
>>>>> + taking its type? */
>>>>> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
>>>>> + {
>>>>> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
>>>>> + if (args
>>>>> + && same_type_p (TREE_VALUE (args), type)
>>>>> + && TREE_CHAIN (args) == void_list_node)
>>>>> + return true;
>>>>> + }
>>>>> + }
>>>>> + return false;
>>>>> +}
>>>>> +
>>>>> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
>>>>> that initializes the LHS (and at least one of its arguments represents
>>>>> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
>>>>> @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr)
>>>>> if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
>>>>> return NULL_TREE;
>>>>> + /* An attempt to reduce the number of -Wdangling-reference
>>>>> + false positives concerning reference wrappers (c++/107532).
>>>>> + Here we suppose that a member function of such a reference
>>>>> + wrapper class returns a reference to a non-temporary object. */
>>>>> + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
>>>>> + && !DECL_OVERLOADED_OPERATOR_P (fndecl)
>>>>> + && reference_like_class_p (CP_DECL_CONTEXT (fndecl)))
>>>>
>>>> Ah, in this case I was thinking rather than return we would want to look
>>>> through to the initializer of the reference wrapper, and warn if that's a
>>>> temporary, so we can catch the *2 cases in your tests.
>>>>
>>>> So, treating ref-like classes as much like references as we can. Some of
>>>> your v1 patch ought to be useful in implementing this, but only looking
>>>> through one call at a time, not all of them like that patch.
>>>
>>> Maybe this one, then? I still have to loop through the calls though; EXPR in
>>> do_warn_dangling_reference can be e.g.
>>>
>>> Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>)
>>>
>>> or
>>>
>>> Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>)
>>>
>>> and we want to warn only about the latter, but that means that I need to
>>> look into the nested call 'planes' to see if the initializer was a temporary.
>>
>> Right, but I was thinking we want to recurse like a few lines above, rather
>> than loop.
>
> Ah yes, I can do that if I introduce a parameter that tells us
> if we're processing an argument or not. I think I'm finally
> more or less satisfied with the patch, thanks.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> This patch adjusts do_warn_dangling_reference so that we look through
> reference wrapper classes (meaning, has a reference member and a
> constructor taking the same reference type, or is std::reference_wrapper
> or std::ranges::ref_view) and don't warn for them, supposing that the
> member function returns a reference to a non-temporary object.
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (reference_like_class_p): New.
> (do_warn_dangling_reference): Add new bool parameter. See through
> reference_like_class_p.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> * g++.dg/warn/Wdangling-reference9.C: New test.
> ---
> gcc/cp/call.cc | 97 +++++++++++++++----
> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++
> .../g++.dg/warn/Wdangling-reference9.C | 21 ++++
> 3 files changed, 178 insertions(+), 17 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index f7c5d9da94b..2a8edc2e7e2 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13777,6 +13777,45 @@ std_pair_ref_ref_p (tree t)
> return true;
> }
>
> +/* Return true if a class CTYPE is either std::reference_wrapper or
> + std::ref_view, or a reference wrapper class. We consider a class
> + a reference wrapper class if it has a reference member and a
> + constructor taking the same reference type. */
> +
> +static bool
> +reference_like_class_p (tree ctype)
> +{
> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> + if (decl_in_std_namespace_p (tdecl))
> + {
> + tree name = DECL_NAME (tdecl);
> + return (name
> + && (id_equal (name, "reference_wrapper")
> + || id_equal (name, "ref_view")));
> + }
> + for (tree fields = TYPE_FIELDS (ctype);
> + fields;
> + fields = DECL_CHAIN (fields))
> + {
> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> + continue;
> + tree type = TREE_TYPE (fields);
> + if (!TYPE_REF_P (type))
> + continue;
> + /* OK, the field is a reference member. Do we have a constructor
> + taking its type? */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> + {
> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> + if (args
> + && same_type_p (TREE_VALUE (args), type)
> + && TREE_CHAIN (args) == void_list_node)
> + return true;
> + }
> + }
> + return false;
> +}
> +
> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> that initializes the LHS (and at least one of its arguments represents
> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
> const int& y = (f(1), 42); // NULL_TREE
> const int& z = f(f(1)); // f(f(1))
>
> - EXPR is the initializer. */
> + EXPR is the initializer. If ARG_P is true, we're processing an argument
> + to a function; the point is to distinguish between, for example,
> +
> + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
> +
> + where we shouldn't warn, and
> +
> + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
> +
> + where we should warn (Ref is a reference_like_class_p so we see through
> + it. */
>
> static tree
> -do_warn_dangling_reference (tree expr)
> +do_warn_dangling_reference (tree expr, bool arg_p)
> {
> STRIP_NOPS (expr);
> + if (TREE_CODE (expr) == ADDR_EXPR)
> + expr = TREE_OPERAND (expr, 0);
I think if we move this here, we also need to check that expr before
STRIP_NOPS had REFERENCE_TYPE. OK with that change.
> + if (arg_p && expr_represents_temporary_p (expr))
> + {
> + /* An attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + Here we suppose that a member function of such a reference
> + wrapper class returns a reference to a non-temporary object. */
> + tree e = expr;
> + while (handled_component_p (e))
> + e = TREE_OPERAND (e, 0);
> + e = TREE_TYPE (e);
> + if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
> + return expr;
> + }
> +
> switch (TREE_CODE (expr))
> {
> case CALL_EXPR:
> @@ -13829,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
> std::pair<const int&, const int&> v = std::minmax(1, 2);
> which also creates a dangling reference, because std::minmax
> returns std::pair<const T&, const T&>(b, a). */
> - if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> + if (!arg_p
> + && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
> return NULL_TREE;
>
> /* Here we're looking to see if any of the arguments is a temporary
> @@ -13842,14 +13909,10 @@ do_warn_dangling_reference (tree expr)
> if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> && !TYPE_REF_P (TREE_TYPE (arg)))
> continue;
> - /* It could also be another call taking a temporary and returning
> - it and initializing this reference parameter. */
> - if (do_warn_dangling_reference (arg))
> - return expr;
> - STRIP_NOPS (arg);
> - if (TREE_CODE (arg) == ADDR_EXPR)
> - arg = TREE_OPERAND (arg, 0);
> - if (expr_represents_temporary_p (arg))
> + /* Recurse to see if the argument is a temporary. It could also
> + be another call taking a temporary and returning it and
> + initializing this reference parameter. */
> + if (do_warn_dangling_reference (arg, /*arg_p=*/true))
> return expr;
> /* Don't warn about member function like:
> std::any a(...);
> @@ -13866,15 +13929,15 @@ do_warn_dangling_reference (tree expr)
> return NULL_TREE;
> }
> case COMPOUND_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
> case COND_EXPR:
> - if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
> + if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
> return t;
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
> case PAREN_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
> case TARGET_EXPR:
> - return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
> + return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
> default:
> return NULL_TREE;
> }
> @@ -13917,7 +13980,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
> = make_temp_override (global_dc->dc_warn_system_headers,
> (!in_system_header_at (DECL_SOURCE_LOCATION (decl))
> || global_dc->dc_warn_system_headers));
> - if (tree call = do_warn_dangling_reference (init))
> + if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
> {
> auto_diagnostic_group d;
> if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..330de1fd05d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner();
> + (void) d8;
> +}
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> new file mode 100644
> index 00000000000..9ad83f7365e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> @@ -0,0 +1,21 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <functional>
> +
> +struct X { int n; };
> +
> +struct S {
> + std::reference_wrapper<const X> wrapit() const { return x; }
> + X x;
> +};
> +
> +void
> +g (const S& s)
> +{
> + const auto& a1 = s.wrapit().get();
> + (void) a1;
> + const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
> + (void) a2;
> +}
>
> base-commit: f661c0bb6371f355966a67b5ce71398e80792948
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-03-01 21:53 ` Jason Merrill
@ 2023-03-02 21:24 ` Marek Polacek
2023-03-03 16:25 ` Jason Merrill
0 siblings, 1 reply; 17+ messages in thread
From: Marek Polacek @ 2023-03-02 21:24 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Wed, Mar 01, 2023 at 04:53:23PM -0500, Jason Merrill wrote:
> > @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
> > const int& y = (f(1), 42); // NULL_TREE
> > const int& z = f(f(1)); // f(f(1))
> > - EXPR is the initializer. */
> > + EXPR is the initializer. If ARG_P is true, we're processing an argument
> > + to a function; the point is to distinguish between, for example,
> > +
> > + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
> > +
> > + where we shouldn't warn, and
> > +
> > + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
> > +
> > + where we should warn (Ref is a reference_like_class_p so we see through
> > + it. */
> > static tree
> > -do_warn_dangling_reference (tree expr)
> > +do_warn_dangling_reference (tree expr, bool arg_p)
> > {
> > STRIP_NOPS (expr);
> > + if (TREE_CODE (expr) == ADDR_EXPR)
> > + expr = TREE_OPERAND (expr, 0);
>
> I think if we move this here, we also need to check that expr before
> STRIP_NOPS had REFERENCE_TYPE. OK with that change.
Sorry but I don't think I can do that. There can be CONVERT_EXPRs
that need to be stripped, whether arg_p or !arg_p. For example, we can get
(const int *) f ((const int &) &TARGET_EXPR <D.2765, NON_LVALUE_EXPR <10>>)
for
const int& r5 = (42, f(10));
Is the patch OK as-is then?
Marek
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-03-02 21:24 ` Marek Polacek
@ 2023-03-03 16:25 ` Jason Merrill
2023-03-03 17:50 ` [PATCH v5] " Marek Polacek
0 siblings, 1 reply; 17+ messages in thread
From: Jason Merrill @ 2023-03-03 16:25 UTC (permalink / raw)
To: Marek Polacek; +Cc: GCC Patches
On 3/2/23 16:24, Marek Polacek wrote:
> On Wed, Mar 01, 2023 at 04:53:23PM -0500, Jason Merrill wrote:
>>> @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
>>> const int& y = (f(1), 42); // NULL_TREE
>>> const int& z = f(f(1)); // f(f(1))
>>> - EXPR is the initializer. */
>>> + EXPR is the initializer. If ARG_P is true, we're processing an argument
>>> + to a function; the point is to distinguish between, for example,
>>> +
>>> + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
>>> +
>>> + where we shouldn't warn, and
>>> +
>>> + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
>>> +
>>> + where we should warn (Ref is a reference_like_class_p so we see through
>>> + it. */
>>> static tree
>>> -do_warn_dangling_reference (tree expr)
>>> +do_warn_dangling_reference (tree expr, bool arg_p)
>>> {
>>> STRIP_NOPS (expr);
>>> + if (TREE_CODE (expr) == ADDR_EXPR)
>>> + expr = TREE_OPERAND (expr, 0);
>>
>> I think if we move this here, we also need to check that expr before
>> STRIP_NOPS had REFERENCE_TYPE. OK with that change.
>
> Sorry but I don't think I can do that. There can be CONVERT_EXPRs
> that need to be stripped, whether arg_p or !arg_p. For example, we can get
> (const int *) f ((const int &) &TARGET_EXPR <D.2765, NON_LVALUE_EXPR <10>>)
> for
> const int& r5 = (42, f(10));
I meant that we only want to strip ADDR_EXPR if 'expr' at the start of
the function had REFERENCE_TYPE, corresponding to
> /* Check that this argument initializes a reference, except for
> the argument initializing the object of a member function. */
> if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> && !TYPE_REF_P (TREE_TYPE (arg)))
> continue;
above the code for stripping an ADDR_EXPR from an argument that your
patch removes.
If the original expr is a pointer rather than a reference, we don't want
to complain about it pointing to a temporary.
Jason
^ permalink raw reply [flat|nested] 17+ messages in thread
* [PATCH v5] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-03-03 16:25 ` Jason Merrill
@ 2023-03-03 17:50 ` Marek Polacek
2023-03-04 2:30 ` Jason Merrill
0 siblings, 1 reply; 17+ messages in thread
From: Marek Polacek @ 2023-03-03 17:50 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Fri, Mar 03, 2023 at 11:25:06AM -0500, Jason Merrill wrote:
> On 3/2/23 16:24, Marek Polacek wrote:
> > On Wed, Mar 01, 2023 at 04:53:23PM -0500, Jason Merrill wrote:
> > > > @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
> > > > const int& y = (f(1), 42); // NULL_TREE
> > > > const int& z = f(f(1)); // f(f(1))
> > > > - EXPR is the initializer. */
> > > > + EXPR is the initializer. If ARG_P is true, we're processing an argument
> > > > + to a function; the point is to distinguish between, for example,
> > > > +
> > > > + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
> > > > +
> > > > + where we shouldn't warn, and
> > > > +
> > > > + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
> > > > +
> > > > + where we should warn (Ref is a reference_like_class_p so we see through
> > > > + it. */
> > > > static tree
> > > > -do_warn_dangling_reference (tree expr)
> > > > +do_warn_dangling_reference (tree expr, bool arg_p)
> > > > {
> > > > STRIP_NOPS (expr);
> > > > + if (TREE_CODE (expr) == ADDR_EXPR)
> > > > + expr = TREE_OPERAND (expr, 0);
> > >
> > > I think if we move this here, we also need to check that expr before
> > > STRIP_NOPS had REFERENCE_TYPE. OK with that change.
> >
> > Sorry but I don't think I can do that. There can be CONVERT_EXPRs
> > that need to be stripped, whether arg_p or !arg_p. For example, we can get
> > (const int *) f ((const int &) &TARGET_EXPR <D.2765, NON_LVALUE_EXPR <10>>)
> > for
> > const int& r5 = (42, f(10));
>
> I meant that we only want to strip ADDR_EXPR if 'expr' at the start of the
> function had REFERENCE_TYPE, corresponding to
>
> > /* Check that this argument initializes a reference, except
> > for
> > the argument initializing the object of a member function. */
> > if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> > && !TYPE_REF_P (TREE_TYPE (arg)))
> > continue;
>
> above the code for stripping an ADDR_EXPR from an argument that your patch
> removes.
I see.
> If the original expr is a pointer rather than a reference, we don't want to
> complain about it pointing to a temporary.
Ug, I can't make it work. When we recurse, I can no longer check
fndecl. How about just moving the stripping back where it was?
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
This patch adjusts do_warn_dangling_reference so that we look through
reference wrapper classes (meaning, has a reference member and a
constructor taking the same reference type, or is std::reference_wrapper
or std::ranges::ref_view) and don't warn for them, supposing that the
member function returns a reference to a non-temporary object.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Add new bool parameter. See through
reference_like_class_p.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
* g++.dg/warn/Wdangling-reference9.C: New test.
---
gcc/cp/call.cc | 92 ++++++++++++++++---
.../g++.dg/warn/Wdangling-reference8.C | 77 ++++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 +++++
3 files changed, 176 insertions(+), 14 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 048b2b052f8..62536573633 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13779,6 +13779,45 @@ std_pair_ref_ref_p (tree t)
return true;
}
+/* Return true if a class CTYPE is either std::reference_wrapper or
+ std::ref_view, or a reference wrapper class. We consider a class
+ a reference wrapper class if it has a reference member and a
+ constructor taking the same reference type. */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+ if (decl_in_std_namespace_p (tdecl))
+ {
+ tree name = DECL_NAME (tdecl);
+ return (name
+ && (id_equal (name, "reference_wrapper")
+ || id_equal (name, "ref_view")));
+ }
+ for (tree fields = TYPE_FIELDS (ctype);
+ fields;
+ fields = DECL_CHAIN (fields))
+ {
+ if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+ continue;
+ tree type = TREE_TYPE (fields);
+ if (!TYPE_REF_P (type))
+ continue;
+ /* OK, the field is a reference member. Do we have a constructor
+ taking its type? */
+ for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
+ {
+ tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+ if (args
+ && same_type_p (TREE_VALUE (args), type)
+ && TREE_CHAIN (args) == void_list_node)
+ return true;
+ }
+ }
+ return false;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13793,12 +13832,37 @@ std_pair_ref_ref_p (tree t)
const int& y = (f(1), 42); // NULL_TREE
const int& z = f(f(1)); // f(f(1))
- EXPR is the initializer. */
+ EXPR is the initializer. If ARG_P is true, we're processing an argument
+ to a function; the point is to distinguish between, for example,
+
+ Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
+
+ where we shouldn't warn, and
+
+ Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
+
+ where we should warn (Ref is a reference_like_class_p so we see through
+ it. */
static tree
-do_warn_dangling_reference (tree expr)
+do_warn_dangling_reference (tree expr, bool arg_p)
{
STRIP_NOPS (expr);
+
+ if (arg_p && expr_represents_temporary_p (expr))
+ {
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Here we suppose that a member function of such a reference
+ wrapper class returns a reference to a non-temporary object. */
+ tree e = expr;
+ while (handled_component_p (e))
+ e = TREE_OPERAND (e, 0);
+ e = TREE_TYPE (e);
+ if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
+ return expr;
+ }
+
switch (TREE_CODE (expr))
{
case CALL_EXPR:
@@ -13831,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
std::pair<const int&, const int&> v = std::minmax(1, 2);
which also creates a dangling reference, because std::minmax
returns std::pair<const T&, const T&>(b, a). */
- if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
+ if (!arg_p
+ && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
return NULL_TREE;
/* Here we're looking to see if any of the arguments is a temporary
@@ -13844,14 +13909,13 @@ do_warn_dangling_reference (tree expr)
if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
&& !TYPE_REF_P (TREE_TYPE (arg)))
continue;
- /* It could also be another call taking a temporary and returning
- it and initializing this reference parameter. */
- if (do_warn_dangling_reference (arg))
- return expr;
STRIP_NOPS (arg);
if (TREE_CODE (arg) == ADDR_EXPR)
arg = TREE_OPERAND (arg, 0);
- if (expr_represents_temporary_p (arg))
+ /* Recurse to see if the argument is a temporary. It could also
+ be another call taking a temporary and returning it and
+ initializing this reference parameter. */
+ if (do_warn_dangling_reference (arg, /*arg_p=*/true))
return expr;
/* Don't warn about member function like:
std::any a(...);
@@ -13868,15 +13932,15 @@ do_warn_dangling_reference (tree expr)
return NULL_TREE;
}
case COMPOUND_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
case COND_EXPR:
- if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
+ if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
return t;
- return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
case PAREN_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
case TARGET_EXPR:
- return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
+ return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
default:
return NULL_TREE;
}
@@ -13919,7 +13983,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
= make_temp_override (global_dc->dc_warn_system_headers,
(!in_system_header_at (DECL_SOURCE_LOCATION (decl))
|| global_dc->dc_warn_system_headers));
- if (tree call = do_warn_dangling_reference (init))
+ if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
{
auto_diagnostic_group d;
if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..330de1fd05d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..9ad83f7365e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+ std::reference_wrapper<const X> wrapit() const { return x; }
+ X x;
+};
+
+void
+g (const S& s)
+{
+ const auto& a1 = s.wrapit().get();
+ (void) a1;
+ const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
+ (void) a2;
+}
base-commit: ce1c99f1ccd7b1229a4f8531d6b6de6cf571a9ef
--
2.39.2
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v5] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-03-03 17:50 ` [PATCH v5] " Marek Polacek
@ 2023-03-04 2:30 ` Jason Merrill
2023-03-06 21:54 ` [PATCH v6] " Marek Polacek
0 siblings, 1 reply; 17+ messages in thread
From: Jason Merrill @ 2023-03-04 2:30 UTC (permalink / raw)
To: Marek Polacek; +Cc: GCC Patches
On 3/3/23 12:50, Marek Polacek wrote:
> On Fri, Mar 03, 2023 at 11:25:06AM -0500, Jason Merrill wrote:
>> On 3/2/23 16:24, Marek Polacek wrote:
>>> On Wed, Mar 01, 2023 at 04:53:23PM -0500, Jason Merrill wrote:
>>>>> @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
>>>>> const int& y = (f(1), 42); // NULL_TREE
>>>>> const int& z = f(f(1)); // f(f(1))
>>>>> - EXPR is the initializer. */
>>>>> + EXPR is the initializer. If ARG_P is true, we're processing an argument
>>>>> + to a function; the point is to distinguish between, for example,
>>>>> +
>>>>> + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
>>>>> +
>>>>> + where we shouldn't warn, and
>>>>> +
>>>>> + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
>>>>> +
>>>>> + where we should warn (Ref is a reference_like_class_p so we see through
>>>>> + it. */
>>>>> static tree
>>>>> -do_warn_dangling_reference (tree expr)
>>>>> +do_warn_dangling_reference (tree expr, bool arg_p)
>>>>> {
>>>>> STRIP_NOPS (expr);
>>>>> + if (TREE_CODE (expr) == ADDR_EXPR)
>>>>> + expr = TREE_OPERAND (expr, 0);
>>>>
>>>> I think if we move this here, we also need to check that expr before
>>>> STRIP_NOPS had REFERENCE_TYPE. OK with that change.
>>>
>>> Sorry but I don't think I can do that. There can be CONVERT_EXPRs
>>> that need to be stripped, whether arg_p or !arg_p. For example, we can get
>>> (const int *) f ((const int &) &TARGET_EXPR <D.2765, NON_LVALUE_EXPR <10>>)
>>> for
>>> const int& r5 = (42, f(10));
>>
>> I meant that we only want to strip ADDR_EXPR if 'expr' at the start of the
>> function had REFERENCE_TYPE, corresponding to
>>
>>> /* Check that this argument initializes a reference, except
>>> for
>>> the argument initializing the object of a member function. */
>>> if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
>>> && !TYPE_REF_P (TREE_TYPE (arg)))
>>> continue;
>>
>> above the code for stripping an ADDR_EXPR from an argument that your patch
>> removes.
>
> I see.
>
>> If the original expr is a pointer rather than a reference, we don't want to
>> complain about it pointing to a temporary.
>
> Ug, I can't make it work. When we recurse, I can no longer check
> fndecl. How about just moving the stripping back where it was?
Sure.
> -- >8 --
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> This patch adjusts do_warn_dangling_reference so that we look through
> reference wrapper classes (meaning, has a reference member and a
> constructor taking the same reference type, or is std::reference_wrapper
> or std::ranges::ref_view) and don't warn for them, supposing that the
> member function returns a reference to a non-temporary object.
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (reference_like_class_p): New.
> (do_warn_dangling_reference): Add new bool parameter. See through
> reference_like_class_p.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> * g++.dg/warn/Wdangling-reference9.C: New test.
> ---
> gcc/cp/call.cc | 92 ++++++++++++++++---
> .../g++.dg/warn/Wdangling-reference8.C | 77 ++++++++++++++++
> .../g++.dg/warn/Wdangling-reference9.C | 21 +++++
> 3 files changed, 176 insertions(+), 14 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 048b2b052f8..62536573633 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13779,6 +13779,45 @@ std_pair_ref_ref_p (tree t)
> return true;
> }
>
> +/* Return true if a class CTYPE is either std::reference_wrapper or
> + std::ref_view, or a reference wrapper class. We consider a class
> + a reference wrapper class if it has a reference member and a
> + constructor taking the same reference type. */
> +
> +static bool
> +reference_like_class_p (tree ctype)
> +{
> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> + if (decl_in_std_namespace_p (tdecl))
> + {
> + tree name = DECL_NAME (tdecl);
> + return (name
> + && (id_equal (name, "reference_wrapper")
> + || id_equal (name, "ref_view")));
> + }
> + for (tree fields = TYPE_FIELDS (ctype);
> + fields;
> + fields = DECL_CHAIN (fields))
> + {
> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> + continue;
> + tree type = TREE_TYPE (fields);
> + if (!TYPE_REF_P (type))
> + continue;
> + /* OK, the field is a reference member. Do we have a constructor
> + taking its type? */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> + {
> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> + if (args
> + && same_type_p (TREE_VALUE (args), type)
> + && TREE_CHAIN (args) == void_list_node)
> + return true;
> + }
> + }
> + return false;
> +}
> +
> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> that initializes the LHS (and at least one of its arguments represents
> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> @@ -13793,12 +13832,37 @@ std_pair_ref_ref_p (tree t)
> const int& y = (f(1), 42); // NULL_TREE
> const int& z = f(f(1)); // f(f(1))
>
> - EXPR is the initializer. */
> + EXPR is the initializer. If ARG_P is true, we're processing an argument
> + to a function; the point is to distinguish between, for example,
> +
> + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
> +
> + where we shouldn't warn, and
> +
> + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
> +
> + where we should warn (Ref is a reference_like_class_p so we see through
> + it. */
>
> static tree
> -do_warn_dangling_reference (tree expr)
> +do_warn_dangling_reference (tree expr, bool arg_p)
> {
> STRIP_NOPS (expr);
> +
> + if (arg_p && expr_represents_temporary_p (expr))
> + {
> + /* An attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + Here we suppose that a member function of such a reference
> + wrapper class returns a reference to a non-temporary object. */
> + tree e = expr;
> + while (handled_component_p (e))
> + e = TREE_OPERAND (e, 0);
> + e = TREE_TYPE (e);
> + if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
> + return expr;
> + }
> +
> switch (TREE_CODE (expr))
> {
> case CALL_EXPR:
> @@ -13831,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
> std::pair<const int&, const int&> v = std::minmax(1, 2);
> which also creates a dangling reference, because std::minmax
> returns std::pair<const T&, const T&>(b, a). */
> - if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> + if (!arg_p
> + && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
Instead of checking !arg_p maybe the std_pair_ref_ref_p call should
change to reference_like_class_p (which in turn should check
std_pair_ref_ref_p)?
> return NULL_TREE;
>
> /* Here we're looking to see if any of the arguments is a temporary
> @@ -13844,14 +13909,13 @@ do_warn_dangling_reference (tree expr)
> if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> && !TYPE_REF_P (TREE_TYPE (arg)))
> continue;
> - /* It could also be another call taking a temporary and returning
> - it and initializing this reference parameter. */
> - if (do_warn_dangling_reference (arg))
> - return expr;
> STRIP_NOPS (arg);
> if (TREE_CODE (arg) == ADDR_EXPR)
> arg = TREE_OPERAND (arg, 0);
> - if (expr_represents_temporary_p (arg))
> + /* Recurse to see if the argument is a temporary. It could also
> + be another call taking a temporary and returning it and
> + initializing this reference parameter. */
> + if (do_warn_dangling_reference (arg, /*arg_p=*/true))
> return expr;
> /* Don't warn about member function like:
> std::any a(...);
> @@ -13868,15 +13932,15 @@ do_warn_dangling_reference (tree expr)
> return NULL_TREE;
> }
> case COMPOUND_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
> case COND_EXPR:
> - if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
> + if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
> return t;
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
> case PAREN_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
> case TARGET_EXPR:
> - return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
> + return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
> default:
> return NULL_TREE;
> }
> @@ -13919,7 +13983,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
> = make_temp_override (global_dc->dc_warn_system_headers,
> (!in_system_header_at (DECL_SOURCE_LOCATION (decl))
> || global_dc->dc_warn_system_headers));
> - if (tree call = do_warn_dangling_reference (init))
> + if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
> {
> auto_diagnostic_group d;
> if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..330de1fd05d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner();
> + (void) d8;
> +}
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> new file mode 100644
> index 00000000000..9ad83f7365e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> @@ -0,0 +1,21 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <functional>
> +
> +struct X { int n; };
> +
> +struct S {
> + std::reference_wrapper<const X> wrapit() const { return x; }
> + X x;
> +};
> +
> +void
> +g (const S& s)
> +{
> + const auto& a1 = s.wrapit().get();
> + (void) a1;
> + const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
> + (void) a2;
> +}
>
> base-commit: ce1c99f1ccd7b1229a4f8531d6b6de6cf571a9ef
^ permalink raw reply [flat|nested] 17+ messages in thread
* [PATCH v6] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-03-04 2:30 ` Jason Merrill
@ 2023-03-06 21:54 ` Marek Polacek
2023-03-07 14:37 ` Jason Merrill
0 siblings, 1 reply; 17+ messages in thread
From: Marek Polacek @ 2023-03-06 21:54 UTC (permalink / raw)
To: Jason Merrill; +Cc: GCC Patches
On Fri, Mar 03, 2023 at 09:30:38PM -0500, Jason Merrill wrote:
> On 3/3/23 12:50, Marek Polacek wrote:
> > switch (TREE_CODE (expr))
> > {
> > case CALL_EXPR:
> > @@ -13831,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
> > std::pair<const int&, const int&> v = std::minmax(1, 2);
> > which also creates a dangling reference, because std::minmax
> > returns std::pair<const T&, const T&>(b, a). */
> > - if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> > + if (!arg_p
> > + && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
>
> Instead of checking !arg_p maybe the std_pair_ref_ref_p call should change
> to reference_like_class_p (which in turn should check std_pair_ref_ref_p)?
Could do. I suppose the logic is that for std::pair<const int&, const int&>
arguments we want to see through it to get at its arguments.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
Here, -Wdangling-reference triggers where it probably shouldn't, causing
some grief. The code in question uses a reference wrapper with a member
function returning a reference to a subobject of a non-temporary object:
const Plane & meta = fm.planes().inner();
I've tried a few approaches, e.g., checking that the member function's
return type is the same as the type of the enclosing class (which is
the case for member functions returning *this), but that then breaks
Wdangling-reference4.C with std::optional<std::string>.
This patch adjusts do_warn_dangling_reference so that we look through
reference wrapper classes (meaning, has a reference member and a
constructor taking the same reference type, or is std::reference_wrapper
or std::ranges::ref_view) and don't warn for them, supposing that the
member function returns a reference to a non-temporary object.
PR c++/107532
gcc/cp/ChangeLog:
* call.cc (reference_like_class_p): New.
(do_warn_dangling_reference): Add new bool parameter. See through
reference_like_class_p.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wdangling-reference8.C: New test.
* g++.dg/warn/Wdangling-reference9.C: New test.
---
gcc/cp/call.cc | 97 ++++++++++++++++---
.../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++
.../g++.dg/warn/Wdangling-reference9.C | 21 ++++
3 files changed, 181 insertions(+), 14 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 048b2b052f8..a43980b6e15 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13779,6 +13779,52 @@ std_pair_ref_ref_p (tree t)
return true;
}
+/* Return true if a class CTYPE is either std::reference_wrapper or
+ std::ref_view, or a reference wrapper class. We consider a class
+ a reference wrapper class if it has a reference member and a
+ constructor taking the same reference type. */
+
+static bool
+reference_like_class_p (tree ctype)
+{
+ if (!CLASS_TYPE_P (ctype))
+ return false;
+
+ /* Also accept a std::pair<const T&, const T&>. */
+ if (std_pair_ref_ref_p (ctype))
+ return true;
+
+ tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
+ if (decl_in_std_namespace_p (tdecl))
+ {
+ tree name = DECL_NAME (tdecl);
+ return (name
+ && (id_equal (name, "reference_wrapper")
+ || id_equal (name, "ref_view")));
+ }
+ for (tree fields = TYPE_FIELDS (ctype);
+ fields;
+ fields = DECL_CHAIN (fields))
+ {
+ if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
+ continue;
+ tree type = TREE_TYPE (fields);
+ if (!TYPE_REF_P (type))
+ continue;
+ /* OK, the field is a reference member. Do we have a constructor
+ taking its type? */
+ for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
+ {
+ tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
+ if (args
+ && same_type_p (TREE_VALUE (args), type)
+ && TREE_CHAIN (args) == void_list_node)
+ return true;
+ }
+ }
+ return false;
+}
+
/* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
that initializes the LHS (and at least one of its arguments represents
a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
@@ -13793,12 +13839,36 @@ std_pair_ref_ref_p (tree t)
const int& y = (f(1), 42); // NULL_TREE
const int& z = f(f(1)); // f(f(1))
- EXPR is the initializer. */
+ EXPR is the initializer. If ARG_P is true, we're processing an argument
+ to a function; the point is to distinguish between, for example,
+
+ Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
+
+ where we shouldn't warn, and
+
+ Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
+
+ where we should warn (Ref is a reference_like_class_p so we see through
+ it. */
static tree
-do_warn_dangling_reference (tree expr)
+do_warn_dangling_reference (tree expr, bool arg_p)
{
STRIP_NOPS (expr);
+
+ if (arg_p && expr_represents_temporary_p (expr))
+ {
+ /* An attempt to reduce the number of -Wdangling-reference
+ false positives concerning reference wrappers (c++/107532).
+ Here we suppose that a member function of such a reference
+ wrapper class returns a reference to a non-temporary object. */
+ tree e = expr;
+ while (handled_component_p (e))
+ e = TREE_OPERAND (e, 0);
+ if (!reference_like_class_p (TREE_TYPE (e)))
+ return expr;
+ }
+
switch (TREE_CODE (expr))
{
case CALL_EXPR:
@@ -13831,7 +13901,7 @@ do_warn_dangling_reference (tree expr)
std::pair<const int&, const int&> v = std::minmax(1, 2);
which also creates a dangling reference, because std::minmax
returns std::pair<const T&, const T&>(b, a). */
- if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
+ if (!(TYPE_REF_OBJ_P (rettype) || reference_like_class_p (rettype)))
return NULL_TREE;
/* Here we're looking to see if any of the arguments is a temporary
@@ -13844,14 +13914,13 @@ do_warn_dangling_reference (tree expr)
if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
&& !TYPE_REF_P (TREE_TYPE (arg)))
continue;
- /* It could also be another call taking a temporary and returning
- it and initializing this reference parameter. */
- if (do_warn_dangling_reference (arg))
- return expr;
STRIP_NOPS (arg);
if (TREE_CODE (arg) == ADDR_EXPR)
arg = TREE_OPERAND (arg, 0);
- if (expr_represents_temporary_p (arg))
+ /* Recurse to see if the argument is a temporary. It could also
+ be another call taking a temporary and returning it and
+ initializing this reference parameter. */
+ if (do_warn_dangling_reference (arg, /*arg_p=*/true))
return expr;
/* Don't warn about member function like:
std::any a(...);
@@ -13868,15 +13937,15 @@ do_warn_dangling_reference (tree expr)
return NULL_TREE;
}
case COMPOUND_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
case COND_EXPR:
- if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
+ if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
return t;
- return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
case PAREN_EXPR:
- return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
+ return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
case TARGET_EXPR:
- return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
+ return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
default:
return NULL_TREE;
}
@@ -13919,7 +13988,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
= make_temp_override (global_dc->dc_warn_system_headers,
(!in_system_header_at (DECL_SOURCE_LOCATION (decl))
|| global_dc->dc_warn_system_headers));
- if (tree call = do_warn_dangling_reference (init))
+ if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
{
auto_diagnostic_group d;
if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
new file mode 100644
index 00000000000..330de1fd05d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
@@ -0,0 +1,77 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+struct Plane { unsigned int bytesused; };
+
+// Passes a reference through. Does not change lifetime.
+template <typename T>
+struct Ref {
+ const T& i_;
+ Ref(const T & i) : i_(i) {}
+ const T & inner();
+};
+
+struct FrameMetadata {
+ Ref<const Plane> planes() const { return p_; }
+
+ Plane p_;
+};
+
+void bar(const Plane & meta);
+void foo(const FrameMetadata & fm)
+{
+ const Plane & meta = fm.planes().inner();
+ bar(meta);
+ const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
+ bar(meta2);
+}
+
+struct S {
+ const S& self () { return *this; }
+} s;
+
+const S& r1 = s.self();
+const S& r2 = S().self(); // { dg-warning "dangling reference" }
+
+struct D {
+};
+
+struct C {
+ D d;
+ Ref<const D> get() const { return d; }
+};
+
+struct B {
+ C c;
+ const C& get() const { return c; }
+ B();
+};
+
+struct A {
+ B b;
+ const B& get() const { return b; }
+};
+
+void
+g (const A& a)
+{
+ const auto& d1 = a.get().get().get().inner();
+ (void) d1;
+ const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d2;
+ const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
+ (void) d3;
+ const auto& d4 = a.b.get().get().inner();
+ (void) d4;
+ const auto& d5 = a.b.c.get().inner();
+ (void) d5;
+ const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
+ (void) d6;
+ Plane p;
+ Ref<Plane> r(p);
+ const auto& d7 = r.inner();
+ (void) d7;
+ const auto& d8 = Ref<Plane>(p).inner();
+ (void) d8;
+}
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
new file mode 100644
index 00000000000..9ad83f7365e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
@@ -0,0 +1,21 @@
+// PR c++/107532
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wdangling-reference" }
+
+#include <functional>
+
+struct X { int n; };
+
+struct S {
+ std::reference_wrapper<const X> wrapit() const { return x; }
+ X x;
+};
+
+void
+g (const S& s)
+{
+ const auto& a1 = s.wrapit().get();
+ (void) a1;
+ const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
+ (void) a2;
+}
base-commit: 553ff2524f412be4e02e2ffb1a0a3dc3e2280742
--
2.39.2
^ permalink raw reply [flat|nested] 17+ messages in thread
* Re: [PATCH v6] c++: -Wdangling-reference with reference wrapper [PR107532]
2023-03-06 21:54 ` [PATCH v6] " Marek Polacek
@ 2023-03-07 14:37 ` Jason Merrill
0 siblings, 0 replies; 17+ messages in thread
From: Jason Merrill @ 2023-03-07 14:37 UTC (permalink / raw)
To: Marek Polacek; +Cc: GCC Patches
On 3/6/23 16:54, Marek Polacek wrote:
> On Fri, Mar 03, 2023 at 09:30:38PM -0500, Jason Merrill wrote:
>> On 3/3/23 12:50, Marek Polacek wrote:
>>> switch (TREE_CODE (expr))
>>> {
>>> case CALL_EXPR:
>>> @@ -13831,7 +13895,8 @@ do_warn_dangling_reference (tree expr)
>>> std::pair<const int&, const int&> v = std::minmax(1, 2);
>>> which also creates a dangling reference, because std::minmax
>>> returns std::pair<const T&, const T&>(b, a). */
>>> - if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
>>> + if (!arg_p
>>> + && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))))
>>
>> Instead of checking !arg_p maybe the std_pair_ref_ref_p call should change
>> to reference_like_class_p (which in turn should check std_pair_ref_ref_p)?
>
> Could do. I suppose the logic is that for std::pair<const int&, const int&>
> arguments we want to see through it to get at its arguments.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> Here, -Wdangling-reference triggers where it probably shouldn't, causing
> some grief. The code in question uses a reference wrapper with a member
> function returning a reference to a subobject of a non-temporary object:
>
> const Plane & meta = fm.planes().inner();
>
> I've tried a few approaches, e.g., checking that the member function's
> return type is the same as the type of the enclosing class (which is
> the case for member functions returning *this), but that then breaks
> Wdangling-reference4.C with std::optional<std::string>.
>
> This patch adjusts do_warn_dangling_reference so that we look through
> reference wrapper classes (meaning, has a reference member and a
> constructor taking the same reference type, or is std::reference_wrapper
> or std::ranges::ref_view) and don't warn for them, supposing that the
> member function returns a reference to a non-temporary object.
>
> PR c++/107532
>
> gcc/cp/ChangeLog:
>
> * call.cc (reference_like_class_p): New.
> (do_warn_dangling_reference): Add new bool parameter. See through
> reference_like_class_p.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wdangling-reference8.C: New test.
> * g++.dg/warn/Wdangling-reference9.C: New test.
> ---
> gcc/cp/call.cc | 97 ++++++++++++++++---
> .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++
> .../g++.dg/warn/Wdangling-reference9.C | 21 ++++
> 3 files changed, 181 insertions(+), 14 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 048b2b052f8..a43980b6e15 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13779,6 +13779,52 @@ std_pair_ref_ref_p (tree t)
> return true;
> }
>
> +/* Return true if a class CTYPE is either std::reference_wrapper or
> + std::ref_view, or a reference wrapper class. We consider a class
> + a reference wrapper class if it has a reference member and a
> + constructor taking the same reference type. */
> +
> +static bool
> +reference_like_class_p (tree ctype)
> +{
> + if (!CLASS_TYPE_P (ctype))
> + return false;
> +
> + /* Also accept a std::pair<const T&, const T&>. */
> + if (std_pair_ref_ref_p (ctype))
> + return true;
> +
> + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype));
> + if (decl_in_std_namespace_p (tdecl))
> + {
> + tree name = DECL_NAME (tdecl);
> + return (name
> + && (id_equal (name, "reference_wrapper")
> + || id_equal (name, "ref_view")));
> + }
> + for (tree fields = TYPE_FIELDS (ctype);
> + fields;
> + fields = DECL_CHAIN (fields))
> + {
> + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields))
> + continue;
> + tree type = TREE_TYPE (fields);
> + if (!TYPE_REF_P (type))
> + continue;
> + /* OK, the field is a reference member. Do we have a constructor
> + taking its type? */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype)))
> + {
> + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn);
> + if (args
> + && same_type_p (TREE_VALUE (args), type)
> + && TREE_CHAIN (args) == void_list_node)
> + return true;
> + }
> + }
> + return false;
> +}
> +
> /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR
> that initializes the LHS (and at least one of its arguments represents
> a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE
> @@ -13793,12 +13839,36 @@ std_pair_ref_ref_p (tree t)
> const int& y = (f(1), 42); // NULL_TREE
> const int& z = f(f(1)); // f(f(1))
>
> - EXPR is the initializer. */
> + EXPR is the initializer. If ARG_P is true, we're processing an argument
> + to a function; the point is to distinguish between, for example,
> +
> + Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
> +
> + where we shouldn't warn, and
> +
> + Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
> +
> + where we should warn (Ref is a reference_like_class_p so we see through
> + it. */
>
> static tree
> -do_warn_dangling_reference (tree expr)
> +do_warn_dangling_reference (tree expr, bool arg_p)
> {
> STRIP_NOPS (expr);
> +
> + if (arg_p && expr_represents_temporary_p (expr))
> + {
> + /* An attempt to reduce the number of -Wdangling-reference
> + false positives concerning reference wrappers (c++/107532).
> + Here we suppose that a member function of such a reference
> + wrapper class returns a reference to a non-temporary object. */
This comment needs updating, I think; OK with that change.
> + tree e = expr;
> + while (handled_component_p (e))
> + e = TREE_OPERAND (e, 0);
> + if (!reference_like_class_p (TREE_TYPE (e)))
> + return expr;
> + }
> +
> switch (TREE_CODE (expr))
> {
> case CALL_EXPR:
> @@ -13831,7 +13901,7 @@ do_warn_dangling_reference (tree expr)
> std::pair<const int&, const int&> v = std::minmax(1, 2);
> which also creates a dangling reference, because std::minmax
> returns std::pair<const T&, const T&>(b, a). */
> - if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))
> + if (!(TYPE_REF_OBJ_P (rettype) || reference_like_class_p (rettype)))
> return NULL_TREE;
>
> /* Here we're looking to see if any of the arguments is a temporary
> @@ -13844,14 +13914,13 @@ do_warn_dangling_reference (tree expr)
> if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> && !TYPE_REF_P (TREE_TYPE (arg)))
> continue;
> - /* It could also be another call taking a temporary and returning
> - it and initializing this reference parameter. */
> - if (do_warn_dangling_reference (arg))
> - return expr;
> STRIP_NOPS (arg);
> if (TREE_CODE (arg) == ADDR_EXPR)
> arg = TREE_OPERAND (arg, 0);
> - if (expr_represents_temporary_p (arg))
> + /* Recurse to see if the argument is a temporary. It could also
> + be another call taking a temporary and returning it and
> + initializing this reference parameter. */
> + if (do_warn_dangling_reference (arg, /*arg_p=*/true))
> return expr;
> /* Don't warn about member function like:
> std::any a(...);
> @@ -13868,15 +13937,15 @@ do_warn_dangling_reference (tree expr)
> return NULL_TREE;
> }
> case COMPOUND_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 1));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p);
> case COND_EXPR:
> - if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1)))
> + if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p))
> return t;
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 2));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p);
> case PAREN_EXPR:
> - return do_warn_dangling_reference (TREE_OPERAND (expr, 0));
> + return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p);
> case TARGET_EXPR:
> - return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr));
> + return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p);
> default:
> return NULL_TREE;
> }
> @@ -13919,7 +13988,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
> = make_temp_override (global_dc->dc_warn_system_headers,
> (!in_system_header_at (DECL_SOURCE_LOCATION (decl))
> || global_dc->dc_warn_system_headers));
> - if (tree call = do_warn_dangling_reference (init))
> + if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false))
> {
> auto_diagnostic_group d;
> if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference,
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> new file mode 100644
> index 00000000000..330de1fd05d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C
> @@ -0,0 +1,77 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +struct Plane { unsigned int bytesused; };
> +
> +// Passes a reference through. Does not change lifetime.
> +template <typename T>
> +struct Ref {
> + const T& i_;
> + Ref(const T & i) : i_(i) {}
> + const T & inner();
> +};
> +
> +struct FrameMetadata {
> + Ref<const Plane> planes() const { return p_; }
> +
> + Plane p_;
> +};
> +
> +void bar(const Plane & meta);
> +void foo(const FrameMetadata & fm)
> +{
> + const Plane & meta = fm.planes().inner();
> + bar(meta);
> + const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" }
> + bar(meta2);
> +}
> +
> +struct S {
> + const S& self () { return *this; }
> +} s;
> +
> +const S& r1 = s.self();
> +const S& r2 = S().self(); // { dg-warning "dangling reference" }
> +
> +struct D {
> +};
> +
> +struct C {
> + D d;
> + Ref<const D> get() const { return d; }
> +};
> +
> +struct B {
> + C c;
> + const C& get() const { return c; }
> + B();
> +};
> +
> +struct A {
> + B b;
> + const B& get() const { return b; }
> +};
> +
> +void
> +g (const A& a)
> +{
> + const auto& d1 = a.get().get().get().inner();
> + (void) d1;
> + const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d2;
> + const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" }
> + (void) d3;
> + const auto& d4 = a.b.get().get().inner();
> + (void) d4;
> + const auto& d5 = a.b.c.get().inner();
> + (void) d5;
> + const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" }
> + (void) d6;
> + Plane p;
> + Ref<Plane> r(p);
> + const auto& d7 = r.inner();
> + (void) d7;
> + const auto& d8 = Ref<Plane>(p).inner();
> + (void) d8;
> +}
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> new file mode 100644
> index 00000000000..9ad83f7365e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C
> @@ -0,0 +1,21 @@
> +// PR c++/107532
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <functional>
> +
> +struct X { int n; };
> +
> +struct S {
> + std::reference_wrapper<const X> wrapit() const { return x; }
> + X x;
> +};
> +
> +void
> +g (const S& s)
> +{
> + const auto& a1 = s.wrapit().get();
> + (void) a1;
> + const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" }
> + (void) a2;
> +}
>
> base-commit: 553ff2524f412be4e02e2ffb1a0a3dc3e2280742
^ permalink raw reply [flat|nested] 17+ messages in thread
end of thread, other threads:[~2023-03-07 14:37 UTC | newest]
Thread overview: 17+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-01-18 17:52 [PATCH] c++: -Wdangling-reference with reference wrapper [PR107532] Marek Polacek
2023-01-18 21:07 ` Jason Merrill
2023-01-19 1:13 ` [PATCH v2] " Marek Polacek
2023-01-19 18:02 ` Jason Merrill
2023-01-20 2:03 ` [PATCH v3] " Marek Polacek
2023-01-20 20:19 ` Jason Merrill
2023-01-24 22:49 ` Marek Polacek
2023-02-06 1:25 ` Jason Merrill
2023-02-07 16:46 ` [PATCH v4] " Marek Polacek
2023-03-01 20:34 ` Marek Polacek
2023-03-01 21:53 ` Jason Merrill
2023-03-02 21:24 ` Marek Polacek
2023-03-03 16:25 ` Jason Merrill
2023-03-03 17:50 ` [PATCH v5] " Marek Polacek
2023-03-04 2:30 ` Jason Merrill
2023-03-06 21:54 ` [PATCH v6] " Marek Polacek
2023-03-07 14:37 ` Jason Merrill
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