Re: [PATCH v3] c++: -Wdangling-reference with reference wrapper [PR107532]

[Jason Merrill <jason@redhat.com>]

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