Re: [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532]

[Marek Polacek <polacek@redhat.com>]

Ping.

On Tue, Feb 07, 2023 at 11:46:10AM -0500, Marek Polacek wrote:
> On Sun, Feb 05, 2023 at 05:25:25PM -0800, Jason Merrill wrote:
> > 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.
> 
> Ah yes, I can do that if I introduce a parameter that tells us
> if we're processing an argument or not.  I think I'm finally
> more or less satisfied with the patch, 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>.
> 
> 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, 178 insertions(+), 17 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 f7c5d9da94b..2a8edc2e7e2 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
> @@ -13791,12 +13830,39 @@ 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 (TREE_CODE (expr) == ADDR_EXPR)
> +    expr = TREE_OPERAND (expr, 0);
> +
> +  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.  */
> +      tree e = expr;
> +      while (handled_component_p (e))
> +	e = TREE_OPERAND (e, 0);
> +      e = TREE_TYPE (e);
> +      if (!CLASS_TYPE_P (e) || !reference_like_class_p (e))
> +	return expr;
> +    }
> +
>    switch (TREE_CODE (expr))
>      {
>      case CALL_EXPR:
> @@ -13829,7 +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))))
>  	  return NULL_TREE;
>  
>  	/* Here we're looking to see if any of the arguments is a temporary
> @@ -13842,14 +13909,10 @@ 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(...);
> @@ -13866,15 +13929,15 @@ 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;
>      }
> @@ -13917,7 +13980,7 @@ 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: f661c0bb6371f355966a67b5ce71398e80792948
> -- 
> 2.39.1
> 

Marek