* [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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