[v2] c++: -Wdangling-reference with reference wrapper [PR107532]

  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


base-commit: 8e2c6e7b426b6c9c13076208b2e176d4aa1432f1
  

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