Re: [PATCH] c++: Implement -Wdangling-reference [PR106393]

[Jason Merrill <jason@redhat.com>]

On 10/21/22 19:28, Marek Polacek wrote:
> This patch implements a new experimental warning (enabled by -Wextra) to
> detect references bound to temporaries whose lifetime has ended.  The

Great!

> primary motivation is the Note in
> <https://en.cppreference.com/w/cpp/algorithm/max>:
> 
>    Capturing the result of std::max by reference produces a dangling reference
>    if one of the parameters is a temporary and that parameter is returned:
> 
>    int n = 1;
>    const int& r = std::max(n-1, n+1); // r is dangling
> 
> That's because both temporaries for n-1 and n+1 are destroyed at the end
> of the full expression.  With this warning enabled, you'll get:
> 
> g.C:3:12: warning: possibly dangling reference to a temporary [-Wdangling-reference]
>      3 | const int& r = std::max(n-1, n+1);
>        |            ^
> g.C:3:24: note: the temporary was destroyed at the end of the full expression 'std::max<int>((n - 1), (n + 1))'
>      3 | const int& r = std::max(n-1, n+1);
>        |                ~~~~~~~~^~~~~~~~~~
> 
> The warning works by checking if a reference is initialized with a function
> that returns a reference, and at least one parameter of the function is
> a reference that is bound to a temporary.  It assumes that such a function
> actually returns one of its arguments!  (I added code to check_return_expr
> to suppress the warning when we've seen the definition of the function
> and we can say that it can return something other than its parameter.)

Hmm, that misses returning a reference to a subobject or container 
element that will also go away when the object is destroyed.  Does it 
also avoid a lot of false positives?

> It doesn't warn when the function in question is a member function, otherwise
> it'd emit loads of warnings for valid code like obj.emplace<T>({0}, 0).

We had discussed warning if the object argument is a temporary (and for 
the above check, the function returns *this)?

> It warns in member initializer lists as well:
> 
>    const int& f(const int& i) { return i; }
>    struct S {
>      const int &r; // { dg-warning "dangling reference" }
>      S() : r(f(10)) { } // { dg-message "destroyed" }
>    };
> 
> I've run the testsuite/bootstrap with the warning enabled by default.
> There were just a few FAILs:
> * g++.dg/warn/Wdangling-pointer-2.C
> * 20_util/any/misc/any_cast.cc
> * 20_util/forward/c_neg.cc
> * 20_util/forward/f_neg.cc
> * experimental/any/misc/any_cast.cc
> all of these look like genuine bugs.  A bootstrap with the warning
> enabled by default passed.
> 
> When testing a previous version of the patch, there were many FAILs in
> libstdc++'s 22_locale/; all of them because the warning triggered on
> 
>    const test_type& obj = std::use_facet<test_type>(std::locale());
> 
> but this code looks valid -- std::use_facet doesn't return a reference
> to its parameter.  Therefore I added code to suppress the warning when
> the call is std::use_facet.  Now 22_locale/* pass even with the warning
> on.  We could exclude more std:: functions like this if desirable.

Instead of adding special cases in the compiler, let's disable the 
warning around the definition of use_facet (and adjust the compiler as 
needed so that avoids the warning).

I was remembering range adaptors being a stated motivation for Nico's 
P2012, but looking back at the paper I now see that this problem was 
avoided for them by disallowing rvalue arguments to range composition.

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/106393
> 
> gcc/c-family/ChangeLog:
> 
> 	* c.opt (Wdangling-reference): New.
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (expr_represents_temporary_p): New, factored out of
> 	conv_binds_ref_to_temporary.
> 	(conv_binds_ref_to_temporary): Don't return false just because a ck_base
> 	is missing.  Use expr_represents_temporary_p.
> 	(find_initializing_call_expr): New.
> 	(do_warn_dangling_reference): New.
> 	(extend_ref_init_temps): Call do_warn_dangling_reference.
> 	* typeck.cc (check_return_expr): Suppress -Wdangling-reference
> 	warnings.
> 
> gcc/ChangeLog:
> 
> 	* doc/invoke.texi: Document -Wdangling-reference.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp23/elision4.C: Use -Wdangling-reference, add dg-warning.
> 	* g++.dg/cpp23/elision7.C: Likewise.
> 	* g++.dg/warn/Wdangling-reference1.C: New test.
> 	* g++.dg/warn/Wdangling-reference2.C: New test.

Could use a test with a virtual base.

> ---
>   gcc/c-family/c.opt                            |   4 +
>   gcc/cp/call.cc                                | 138 ++++++++++++++++--
>   gcc/cp/cp-tree.h                              |   4 +-
>   gcc/cp/typeck.cc                              |  10 ++
>   gcc/doc/invoke.texi                           |  34 ++++-
>   gcc/testsuite/g++.dg/cpp23/elision4.C         |   5 +-
>   gcc/testsuite/g++.dg/cpp23/elision7.C         |   3 +-
>   .../g++.dg/warn/Wdangling-reference1.C        | 103 +++++++++++++
>   .../g++.dg/warn/Wdangling-reference2.C        |  28 ++++
>   9 files changed, 312 insertions(+), 17 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference2.C
> 
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 01d480759ae..02d79991aeb 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -555,6 +555,10 @@ Wdangling-pointer=
>   C ObjC C++ ObjC++ Joined RejectNegative UInteger Var(warn_dangling_pointer) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall, 2, 0) IntegerRange(0, 2)
>   Warn for uses of pointers to auto variables whose lifetime has ended.
>   
> +Wdangling-reference
> +C++ ObjC++ Var(warn_dangling_reference) Warning LangEnabledBy(C++ ObjC++, Wextra)
> +Warn when a reference is bound to a temporary whose lifetime has ended.
> +
>   Wdate-time
>   C ObjC C++ ObjC++ CPP(warn_date_time) CppReason(CPP_W_DATE_TIME) Var(cpp_warn_date_time) Init(0) Warning
>   Warn about __TIME__, __DATE__ and __TIMESTAMP__ usage.
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 6a34e9c2ae1..43e607987a0 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -9313,6 +9313,16 @@ conv_binds_ref_to_prvalue (conversion *c)
>     return conv_is_prvalue (next_conversion (c));
>   }
>   
> +/* True iff EXPR represents a (subobject of a) temporary.  */
> +
> +static bool
> +expr_represents_temporary_p (tree expr)
> +{
> +  while (handled_component_p (expr))
> +    expr = TREE_OPERAND (expr, 0);
> +  return TREE_CODE (expr) == TARGET_EXPR;
> +}
> +
>   /* True iff C is a conversion that binds a reference to a temporary.
>      This is a superset of conv_binds_ref_to_prvalue: here we're also
>      interested in xvalues.  */
> @@ -9330,18 +9340,14 @@ conv_binds_ref_to_temporary (conversion *c)
>          struct Derived : Base {};
>          const Base& b(Derived{});
>        where we bind 'b' to the Base subobject of a temporary object of type
> -     Derived.  The subobject is an xvalue; the whole object is a prvalue.  */
> -  if (c->kind != ck_base)
> -    return false;
> -  c = next_conversion (c);
> -  if (c->kind == ck_identity && c->u.expr)
> -    {
> -      tree expr = c->u.expr;
> -      while (handled_component_p (expr))
> -	expr = TREE_OPERAND (expr, 0);
> -      if (TREE_CODE (expr) == TARGET_EXPR)
> -	return true;
> -    }
> +     Derived.  The subobject is an xvalue; the whole object is a prvalue.
> +
> +     The ck_base doesn't have to be present for cases like X{}.m.  */
> +  if (c->kind == ck_base)
> +    c = next_conversion (c);
> +  if (c->kind == ck_identity && c->u.expr
> +      && expr_represents_temporary_p (c->u.expr))
> +    return true;
>     return false;
>   }
>   
> @@ -13428,6 +13434,111 @@ initialize_reference (tree type, tree expr,
>     return expr;
>   }
>   
> +/* Helper for do_warn_dangling_reference to find a non-nested CALL_EXPR
> +   that initializes the LHS, or NULL_TREE if none found.  For instance:
> +
> +     const int& r = (42, f(1)); // f(1)
> +     const int& t = b ? f(1) : f(2); // f(1)
> +     const int& z = (f(1), 42); // NULL_TREE
> +
> +   EXPR is the initializer.  */
> +
> +static tree
> +find_initializing_call_expr (tree expr)
> +{
> +  STRIP_NOPS (expr);
> +  switch (TREE_CODE (expr))
> +    {
> +    case CALL_EXPR:
> +      return expr;
> +    case COMPOUND_EXPR:
> +      return find_initializing_call_expr (TREE_OPERAND (expr, 1));
> +    case COND_EXPR:
> +      if (tree t = find_initializing_call_expr (TREE_OPERAND (expr, 1)))
> +	return t;
> +      return find_initializing_call_expr (TREE_OPERAND (expr, 2));

For COND_EXPR I think we want to check both sides, in case there are 
calls on both sides but only the second one has a problematic temporary.

> +    case PAREN_EXPR:
> +      return find_initializing_call_expr (TREE_OPERAND (expr, 0));
> +    default:
> +      return NULL_TREE;
> +    }
> +}
> +
> +/* Implement -Wdangling-reference, to detect cases like
> +
> +     int n = 1;
> +     const int& r = std::max(n - 1, n + 1); // r is dangling
> +
> +   This creates temporaries from the arguments, returns a reference to
> +   one of the temporaries, but both temporaries are destroyed at the end
> +   of the full expression.
> +
> +   This works by checking if a reference is initialized with a function
> +   that returns a reference, and at least one parameter of the function
> +   is a reference that is bound to a temporary.  It assumes that such a
> +   function actually returns one of its arguments.
> +
> +   This warning doesn't warn when the function in question is a member
> +   function.
> +
> +   DECL is the reference being initialized, CALL is the initializer.  */
> +
> +static void
> +do_warn_dangling_reference (const_tree decl, tree call)
> +{
> +  if (!warn_dangling_reference)
> +    return;
> +  if (!TYPE_REF_P (TREE_TYPE (decl)))
> +    return;
> +  call = find_initializing_call_expr (call);
> +  if (call == NULL_TREE)
> +    return;
> +
> +  tree fndecl = cp_get_callee_fndecl_nofold (call);
> +  if (!fndecl
> +      || warning_suppressed_p (fndecl, OPT_Wdangling_reference)
> +      /* Don't warn about member functions; the warning would trigger in
> +	 valid code like
> +	   std::any a(...);
> +	   S& s = a.emplace<S>({0}, 0);
> +	 which constructs a new object and returns a reference to it.  */
> +      || DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> +      /* It seems unreasonable to warn about operator functions.  */
> +      || DECL_OVERLOADED_OPERATOR_P (fndecl)

I guess I'd expect false positives on << and >> because of iostreams, do 
you see false positives with other operators?

> +      /* If the function doesn't return a reference, don't warn.  This can
> +	 be e.g.
> +	   const int& z = std::min({1, 2, 3, 4, 5, 6, 7});
> +	 which doesn't dangle: std::min here returns an int.  */
> +      || !TYPE_REF_P (TREE_TYPE (TREE_TYPE (fndecl))))
> +    return;
> +
> +  /* Mitigate some known cases.  */
> +  if (decl_in_std_namespace_p (fndecl))
> +    if (tree name = DECL_NAME (fndecl))
> +      if (id_equal (name, "use_facet"))
> +	return;
> +
> +  for (int i = 0; i < call_expr_nargs (call); ++i)
> +    {
> +      /* We're looking to see if ARG is something like
> +	  (const int &) &TARGET_EXPR <...>.  */
> +      tree arg = CALL_EXPR_ARG (call, i);
> +      STRIP_NOPS (arg);
> +      if (TREE_CODE (arg) == ADDR_EXPR)
> +	arg = TREE_OPERAND (arg, 0);
> +      if (expr_represents_temporary_p (arg))
> +	{
> +	  auto_diagnostic_group d;
> +	  if (warning_at (DECL_SOURCE_LOCATION (decl),
> +			  OPT_Wdangling_reference,
> +			  "possibly dangling reference to a temporary"))
> +	    inform (EXPR_LOCATION (call), "the temporary was destroyed at "
> +		    "the end of the full expression %qE", call);
> +	  return;
> +	}
> +    }
> +}
> +
>   /* If *P is an xvalue expression, prevent temporary lifetime extension if it
>      gets used to initialize a reference.  */
>   
> @@ -13525,6 +13636,9 @@ extend_ref_init_temps (tree decl, tree init, vec<tree, va_gc> **cleanups,
>     tree type = TREE_TYPE (init);
>     if (processing_template_decl)
>       return init;
> +
> +  do_warn_dangling_reference (decl, init);
> +
>     if (TYPE_REF_P (type))
>       init = extend_ref_init_temps_1 (decl, init, cleanups, cond_guard);
>     else
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 60a25101049..8ddf55ce2b0 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -459,7 +459,6 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
>         TI_PENDING_TEMPLATE_FLAG.
>         TEMPLATE_PARMS_FOR_INLINE.
>         DELETE_EXPR_USE_VEC (in DELETE_EXPR).
> -      (TREE_CALLS_NEW) (in _EXPR or _REF) (commented-out).
>         ICS_ELLIPSIS_FLAG (in _CONV)
>         DECL_INITIALIZED_P (in VAR_DECL)
>         TYPENAME_IS_CLASS_P (in TYPENAME_TYPE)
> @@ -4558,6 +4557,9 @@ get_vec_init_expr (tree t)
>      When appearing in a CONSTRUCTOR, the expression is an unconverted
>      compound literal.
>   
> +   When appearing in a CALL_EXPR, it means that it is a call to
> +   a constructor.
> +
>      When appearing in a FIELD_DECL, it means that this field
>      has been duly initialized in its constructor.  */
>   #define TREE_HAS_CONSTRUCTOR(NODE) (TREE_LANG_FLAG_4 (NODE))
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 16e7d85793d..5a22eee8ebf 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -11238,6 +11238,16 @@ check_return_expr (tree retval, bool *no_warning)
>     if (processing_template_decl)
>       return saved_retval;
>   
> +  /* A naive attempt to reduce the number of -Wdangling-reference false
> +     positives: if we know that this function can return something other
> +     than one of its parameters, suppress the warning.  */
> +  if (warn_dangling_reference
> +      && TYPE_REF_P (functype)
> +      && bare_retval
> +      && (!REFERENCE_REF_P (bare_retval)
> +	  || TREE_CODE (TREE_OPERAND (bare_retval, 0)) != PARM_DECL))
> +    suppress_warning (current_function_decl, OPT_Wdangling_reference);
> +
>     /* Actually copy the value returned into the appropriate location.  */
>     if (retval && retval != result)
>       retval = cp_build_init_expr (result, retval);
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 09548c4528c..28bb395ce6c 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -249,7 +249,8 @@ in the following sections.
>   -Wno-class-conversion  -Wclass-memaccess @gol
>   -Wcomma-subscript  -Wconditionally-supported @gol
>   -Wno-conversion-null  -Wctad-maybe-unsupported @gol
> --Wctor-dtor-privacy  -Wno-delete-incomplete @gol
> +-Wctor-dtor-privacy  -Wdangling-reference @gol
> +-Wno-delete-incomplete @gol
>   -Wdelete-non-virtual-dtor  -Wno-deprecated-array-compare @gol
>   -Wdeprecated-copy -Wdeprecated-copy-dtor @gol
>   -Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion @gol
> @@ -3627,6 +3628,36 @@ public static member functions.  Also warn if there are no non-private
>   methods, and there's at least one private member function that isn't
>   a constructor or destructor.
>   
> +@item -Wdangling-reference @r{(C++ and Objective-C++ only)}
> +@opindex Wdangling-reference
> +@opindex Wno-dangling-reference
> +Warn when a reference is bound to a temporary whose lifetime has ended.
> +For example:
> +
> +@smallexample
> +int n = 1;
> +const int& r = std::max(n - 1, n + 1); // r is dangling
> +@end smallexample
> +
> +In the example above, two temporaries are created, one for each
> +argument, and a reference to one of the temporaries is returned.
> +However, both temporaries are destroyed at the end of the full
> +expression, so the reference @code{r} is dangling.  This warning
> +also detects dangling references in member initializer lists:
> +
> +@smallexample
> +const int& f(const int& i) @{ return i; @}
> +struct S @{
> +  const int &r; // r is dangling
> +  S() : r(f(10)) @{ @}
> +@};
> +@end smallexample
> +
> +Member functions are not checked.  Certain standard functions, such
> +as @code{std::use_facet}, are also excluded from checking.
> +
> +This warning is enabled by @option{-Wextra}.
> +
>   @item -Wdelete-non-virtual-dtor @r{(C++ and Objective-C++ only)}
>   @opindex Wdelete-non-virtual-dtor
>   @opindex Wno-delete-non-virtual-dtor
> @@ -5936,6 +5967,7 @@ name is still supported, but the newer name is more descriptive.)
>   
>   @gccoptlist{-Wclobbered  @gol
>   -Wcast-function-type  @gol
> +-Wdangling-reference @r{(C++ only)} @gol
>   -Wdeprecated-copy @r{(C++ only)} @gol
>   -Wempty-body  @gol
>   -Wenum-conversion @r{(C only)} @gol
> diff --git a/gcc/testsuite/g++.dg/cpp23/elision4.C b/gcc/testsuite/g++.dg/cpp23/elision4.C
> index c19b86b8b5f..d39053ad741 100644
> --- a/gcc/testsuite/g++.dg/cpp23/elision4.C
> +++ b/gcc/testsuite/g++.dg/cpp23/elision4.C
> @@ -1,5 +1,6 @@
>   // PR c++/101165 - P2266R1 - Simpler implicit move
>   // { dg-do compile { target c++23 } }
> +// { dg-options "-Wdangling-reference" }
>   // Test from P2266R1, $ 5.2. LibreOffice OString constructor.
>   
>   struct X {
> @@ -33,6 +34,6 @@ T& temporary2(T&& x) { return static_cast<T&>(x); }
>   void
>   test ()
>   {
> -  int& r1 = temporary1 (42);
> -  int& r2 = temporary2 (42);
> +  int& r1 = temporary1 (42); // { dg-warning "dangling reference" }
> +  int& r2 = temporary2 (42); // { dg-warning "dangling reference" }
>   }
> diff --git a/gcc/testsuite/g++.dg/cpp23/elision7.C b/gcc/testsuite/g++.dg/cpp23/elision7.C
> index 19fa89ae133..0045842b34f 100644
> --- a/gcc/testsuite/g++.dg/cpp23/elision7.C
> +++ b/gcc/testsuite/g++.dg/cpp23/elision7.C
> @@ -1,5 +1,6 @@
>   // PR c++/101165 - P2266R1 - Simpler implicit move
>   // { dg-do compile { target c++23 } }
> +// { dg-options "-Wdangling-reference" }
>   
>   struct X {
>     X ();
> @@ -68,5 +69,5 @@ f7 (T &&t)
>   void
>   do_f7 ()
>   {
> -  const int &x = f7 (0);
> +  const int &x = f7 (0); // { dg-warning "dangling reference" }
>   }
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> new file mode 100644
> index 00000000000..e0324d38f67
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C
> @@ -0,0 +1,103 @@
> +// PR c++/106393
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +const int& f(const int& i) { return i; }
> +const int& h(int);
> +int g;
> +const int& globref(const int&) { return g; }
> +struct X {
> +  int* i;
> +  operator const int&() const { return *i; }
> +};
> +X x{&g};
> +
> +const int& r1 = f(10); // { dg-warning "dangling reference" }
> +// r2 = _ZGR2r2_ = (int) *f ((const int &) &TARGET_EXPR <D.2429, 10>) + 1; (const int &) &_ZGR2r2_
> +const int& r2 = f(10) + 1;
> +// Don't warn here, we have
> +//   r3 = f (X::operator const int& (&x))
> +const int& r3 = f(x);
> +// Don't warn here, because we've seen the definition of globref
> +// and could figure out that it may not return one of its parms.
> +// Questionable -- it can also hide bugs --, but it helps here.

We could suppress specifically for the case of returning a variable with 
static storage duration?

> +const int& r4 = globref(1);
> +const int& r5 = (42, f(10)); // { dg-warning "dangling reference" }
> +const int& r6 = (f(10), 42);
> +const int& r7 = (f(10)); // { dg-warning "dangling reference" }
> +const int& r8 = g ? f(10) : f(9); // { dg-warning "dangling reference" }
> +const int& r9 = (42, g ? f(10) : f(9)); // { dg-warning "dangling reference" }
> +const int& r10 = (g ? f(10) : f(9), 42);
> +// Binds to a reference temporary for r11.
> +const int& r11 = g ? f(10) : 9;

Why no warning?

> +// Invalid, but we don't warn here yet.
> +// r12 = f (f ((const int &) &TARGET_EXPR <D.2459, 1>))
> +const int& r12 = f(f(1));

This should be a simple recursion?

> +const int& r13 = h(f(1));
> +// Other forms of initializers.
> +const int& r14(f(10)); // { dg-warning "dangling reference" }
> +const int& r15(f(10)); // { dg-warning "dangling reference" }
> +// Returns a ref, but doesn't have a parameter of reference type.
> +const int& r16 = h(10);
> +
> +// OK: the reference is bound to the 10 so still valid at the point
> +// where it's copied into i1.
> +int i1 = f(10);
> +
> +int
> +test1 ()
> +{
> +  const int &lr = f(10); // { dg-warning "dangling reference" }
> +  int i2 = f(10);
> +  return lr;
> +}
> +
> +struct B { };
> +struct D : B { };
> +struct C {
> +  D d;
> +};
> +
> +C c;
> +D d;
> +
> +using U = D[3];
> +
> +const B& frotz(const D&);
> +const B& b1 = frotz(C{}.d); // { dg-warning "dangling reference" }
> +const B& b2 = frotz(D{}); // { dg-warning "dangling reference" }
> +const B& b3 = frotz(c.d);
> +const B& b4 = frotz(d);
> +const B& b5 = frotz(U{}[0]); // { dg-warning "dangling reference" }
> +
> +struct E {
> +  E(int);
> +};
> +const E& operator*(const E&);
> +const E& b6 = *E(1);
> +
> +struct S {
> +  const int &r; // { dg-warning "dangling reference" }
> +  S() : r(f(10)) { } // { dg-message "destroyed" }
> +};
> +
> +// From cppreference.
> +template<class T>
> +const T& max(const T& a, const T& b)
> +{
> +    return (a < b) ? b : a;
> +}
> +
> +int n = 1;
> +const int& r20 = max(n - 1, n + 1); // { dg-warning "dangling reference" }
> +
> +// Don't warn about member functions.
> +struct Y {
> +  operator int&&();
> +  const int& foo(const int&);
> +};
> +
> +// x1 = Y::operator int&& (&TARGET_EXPR <D.2410, {}>)
> +int&& x1 = Y();
> +int&& x2 = Y{};
> +const int& t1 = Y().foo(10);
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference2.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference2.C
> new file mode 100644
> index 00000000000..dafdb43f1b9
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference2.C
> @@ -0,0 +1,28 @@
> +// PR c++/106393
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wdangling-reference" }
> +
> +namespace std {
> +struct any {};
> +template <typename _ValueType> _ValueType any_cast(any &&);
> +template <typename _Tp> struct remove_reference { using type = _Tp; };
> +template <typename _Tp> _Tp forward(typename remove_reference<_Tp>::type);
> +template <typename _Tp> typename remove_reference<_Tp>::type move(_Tp);
> +} // namespace std
> +
> +const int &r = std::any_cast<int&>(std::any()); // { dg-warning "dangling reference" }
> +
> +template <class T> struct C {
> +  T t_; // { dg-warning "dangling reference" }
> +  C(T);
> +  template <class U> C(U c) : t_(std::forward<T>(c.t_)) {}
> +};
> +struct A {};
> +struct B {
> +  B(A);
> +};
> +int main() {
> +  A a;
> +  C<A> ca(a);
> +  C<B &&>(std::move(ca));
> +}
> 
> base-commit: 5792208f5124f687376f25798668d105d7ddb270