Re: [PATCH v6] c++: -Wdangling-reference with reference wrapper [PR107532]

[Jason Merrill <jason@redhat.com>]

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