From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH v2] c++: -Wdangling-reference with reference wrapper [PR107532]
Date: Thu, 19 Jan 2023 13:02:02 -0500 [thread overview]
Message-ID: <c1a89197-7cd1-b7d9-1473-e94f5443c0ac@redhat.com> (raw)
In-Reply-To: <Y8iZOKnDx+14BjOD@redhat.com>
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
next prev parent reply other threads:[~2023-01-19 18:02 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-18 17:52 [PATCH] " 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 [this message]
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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