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

[Jason Merrill <jason@redhat.com>]

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