Re: [PATCH] c++: Add __reference_con{struc,ver}ts_from_temporary [PR104477]

[Jason Merrill <jason@redhat.com>]

On 7/8/22 13:41, Marek Polacek wrote:
> This patch implements C++23 P2255R2, which adds two new type traits to
> detect reference binding to a temporary.  They can be used to detect code
> like
> 
>    std::tuple<const std::string&> t("meow");
> 
> which is incorrect because it always creates a dangling reference, because
> the std::string temporary is created inside the selected constructor of
> std::tuple, and not outside it.
> 
> There are two new compiler builtins, __reference_constructs_from_temporary
> and __reference_converts_from_temporary.  The former is used to simulate
> direct- and the latter copy-initialization context.  But I had a hard time
> finding a test where there's actually a difference.  Under DR 2267, both
> of these are invalid:
> 
>    struct A { } a;
>    struct B { explicit B(const A&); };
>    const B &b1{a};
>    const B &b2(a);
> 
> so I had to peruse [over.match.ref], and eventually realized that the
> difference can be seen here:
> 
>    struct G {
>      operator int(); // #1
>      explicit operator int&&(); // #2
>    };
> 
> int&& r1(G{}); // use #2 (no temporary)
> int&& r2 = G{}; // use #1 (a temporary is created to be bound to int&&)
> 
> The implementation itself was rather straightforward because we already
> have conv_binds_ref_to_prvalue.  The main function here is
> reference_from_temporary.  The renaming to ref_conv_binds_to_temporary_p
> is because previously the function didn't distinguish between an invalid
> conversion and one that binds to a prvalue.
> 
> The patch also adds the relevant class and variable templates to <type_traits>.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> 	PR c++/104477
> 
> gcc/c-family/ChangeLog:
> 
> 	* c-common.cc (c_common_reswords): Add
> 	__reference_constructs_from_temporary and
> 	__reference_converts_from_temporary.
> 	* c-common.h (enum rid): Add RID_REF_CONSTRUCTS_FROM_TEMPORARY and
> 	RID_REF_CONVERTS_FROM_TEMPORARY.
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (ref_conv_binds_directly_p): Rename to ...
> 	(ref_conv_binds_to_temporary_p): ... this.  Add a new bool
> 	parameter.  Return true only if the conversion is valid and
> 	conv_binds_ref_to_prvalue returns true.
> 	* constraint.cc (diagnose_trait_expr): Handle
> 	CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and CPTK_REF_CONVERTS_FROM_TEMPORARY.
> 	* cp-tree.h (enum cp_trait_kind): Add CPTK_REF_CONSTRUCTS_FROM_TEMPORARY
> 	and CPTK_REF_CONVERTS_FROM_TEMPORARY.
> 	(ref_conv_binds_directly_p): Rename to ...
> 	(ref_conv_binds_to_temporary_p): ... this.
> 	(reference_from_temporary): Declare.
> 	* cxx-pretty-print.cc (pp_cxx_trait_expression): Handle
> 	CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and CPTK_REF_CONVERTS_FROM_TEMPORARY.
> 	* method.cc (reference_from_temporary): New.
> 	* parser.cc (cp_parser_primary_expression): Handle
> 	RID_REF_CONSTRUCTS_FROM_TEMPORARY and RID_REF_CONVERTS_FROM_TEMPORARY.
> 	(cp_parser_trait_expr): Likewise.
> 	(warn_for_range_copy): Adjust to call ref_conv_binds_to_temporary_p.
> 	* semantics.cc (trait_expr_value): Handle
> 	CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and CPTK_REF_CONVERTS_FROM_TEMPORARY.
> 	(finish_trait_expr): Likewise.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/type_traits (reference_constructs_from_temporary,
> 	reference_converts_from_temporary): New class templates.
> 	(reference_constructs_from_temporary_v,
> 	reference_converts_from_temporary_v): New variable templates.
> 	(__cpp_lib_reference_from_temporary): Define for C++23.
> 	* include/std/version (__cpp_lib_reference_from_temporary): Define for
> 	C++23.
> 	* testsuite/20_util/variable_templates_for_traits.cc: Test
> 	reference_constructs_from_temporary_v and
> 	reference_converts_from_temporary_v.
> 	* testsuite/20_util/reference_from_temporary/value.cc: New test.
> 	* testsuite/20_util/reference_from_temporary/value2.cc: New test.
> 	* testsuite/20_util/reference_from_temporary/version.cc: New test.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/ext/reference_constructs_from_temporary1.C: New test.
> 	* g++.dg/ext/reference_converts_from_temporary1.C: New test.
> ---
>   gcc/c-family/c-common.cc                      |   4 +
>   gcc/c-family/c-common.h                       |   2 +
>   gcc/cp/call.cc                                |  14 +-
>   gcc/cp/constraint.cc                          |   8 +
>   gcc/cp/cp-tree.h                              |   7 +-
>   gcc/cp/cxx-pretty-print.cc                    |   6 +
>   gcc/cp/method.cc                              |  28 +++
>   gcc/cp/parser.cc                              |  14 +-
>   gcc/cp/semantics.cc                           |   8 +
>   .../reference_constructs_from_temporary1.C    | 214 ++++++++++++++++++
>   .../ext/reference_converts_from_temporary1.C  | 214 ++++++++++++++++++
>   libstdc++-v3/include/std/type_traits          |  39 ++++
>   libstdc++-v3/include/std/version              |   5 +-
>   .../20_util/reference_from_temporary/value.cc | 110 +++++++++
>   .../reference_from_temporary/value2.cc        |  28 +++
>   .../reference_from_temporary/version.cc       |  27 +++
>   .../20_util/variable_templates_for_traits.cc  |  14 ++
>   17 files changed, 730 insertions(+), 12 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
>   create mode 100644 gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
>   create mode 100644 libstdc++-v3/testsuite/20_util/reference_from_temporary/value.cc
>   create mode 100644 libstdc++-v3/testsuite/20_util/reference_from_temporary/value2.cc
>   create mode 100644 libstdc++-v3/testsuite/20_util/reference_from_temporary/version.cc
> 
> diff --git a/gcc/c-family/c-common.cc b/gcc/c-family/c-common.cc
> index 1b8e73f7bc5..655c3aefee6 100644
> --- a/gcc/c-family/c-common.cc
> +++ b/gcc/c-family/c-common.cc
> @@ -537,6 +537,10 @@ const struct c_common_resword c_common_reswords[] =
>     { "__is_constructible", RID_IS_CONSTRUCTIBLE, D_CXXONLY },
>     { "__is_nothrow_assignable", RID_IS_NOTHROW_ASSIGNABLE, D_CXXONLY },
>     { "__is_nothrow_constructible", RID_IS_NOTHROW_CONSTRUCTIBLE, D_CXXONLY },
> +  { "__reference_constructs_from_temporary", RID_REF_CONSTRUCTS_FROM_TEMPORARY,
> +					D_CXXONLY },
> +  { "__reference_converts_from_temporary", RID_REF_CONVERTS_FROM_TEMPORARY,
> +					D_CXXONLY },
>   
>     /* C++ transactional memory.  */
>     { "synchronized",	RID_SYNCHRONIZED, D_CXX_OBJC | D_TRANSMEM },
> diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
> index c0900848965..f9064393b4e 100644
> --- a/gcc/c-family/c-common.h
> +++ b/gcc/c-family/c-common.h
> @@ -184,6 +184,8 @@ enum rid
>     RID_IS_UNION,                RID_UNDERLYING_TYPE,
>     RID_IS_ASSIGNABLE,           RID_IS_CONSTRUCTIBLE,
>     RID_IS_NOTHROW_ASSIGNABLE,   RID_IS_NOTHROW_CONSTRUCTIBLE,
> +  RID_REF_CONSTRUCTS_FROM_TEMPORARY,
> +  RID_REF_CONVERTS_FROM_TEMPORARY,
>   
>     /* C++11 */
>     RID_CONSTEXPR, RID_DECLTYPE, RID_NOEXCEPT, RID_NULLPTR, RID_STATIC_ASSERT,
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index fc98552fda2..1ba209f61f1 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -9109,21 +9109,23 @@ conv_binds_ref_to_prvalue (conversion *c)
>     return conv_is_prvalue (next_conversion (c));
>   }
>   
> -/* True iff converting EXPR to a reference type TYPE does not involve
> -   creating a temporary.  */
> +/* True iff converting EXPR to a reference type TYPE binds the reference to
> +   a temporary.  DIRECT_INIT_P says whether the conversion should be done
> +   in direct- or copy-initialization context.  */
>   
>   bool
> -ref_conv_binds_directly_p (tree type, tree expr)
> +ref_conv_binds_to_temporary_p (tree type, tree expr,
> +			       bool direct_init_p /*= false*/)
>   {
>     gcc_assert (TYPE_REF_P (type));
>   
>     /* Get the high-water mark for the CONVERSION_OBSTACK.  */
>     void *p = conversion_obstack_alloc (0);
>   
> +  const int flags = direct_init_p ? LOOKUP_NORMAL : LOOKUP_IMPLICIT;
>     conversion *conv = implicit_conversion (type, TREE_TYPE (expr), expr,
> -					  /*c_cast_p=*/false,
> -					  LOOKUP_IMPLICIT, tf_none);
> -  bool ret = conv && !conv->bad_p && !conv_binds_ref_to_prvalue (conv);
> +					  /*c_cast_p=*/false, flags, tf_none);
> +  bool ret = conv && !conv->bad_p && conv_binds_ref_to_prvalue (conv);
>   
>     /* Free all the conversions we allocated.  */
>     obstack_free (&conversion_obstack, p);
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index 591155cee22..648cc9d176d 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -3687,6 +3687,14 @@ diagnose_trait_expr (tree expr, tree args)
>       case CPTK_HAS_UNIQUE_OBJ_REPRESENTATIONS:
>         inform (loc, "  %qT does not have unique object representations", t1);
>         break;
> +    case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
> +      inform (loc, "  %qT is not a reference that binds to a temporary "
> +	      "object of type %qT (direct-initialization)", t1, t2);
> +      break;
> +    case CPTK_REF_CONVERTS_FROM_TEMPORARY:
> +      inform (loc, "  %qT is not a reference that binds to a temporary "
> +	      "object of type %qT (copy-initialization)", t1, t2);
> +      break;
>       case CPTK_BASES:
>       case CPTK_DIRECT_BASES:
>       case CPTK_UNDERLYING_TYPE:
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 2fde4f83b41..c3bed31e455 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -1397,7 +1397,9 @@ enum cp_trait_kind
>     CPTK_IS_ASSIGNABLE,
>     CPTK_IS_CONSTRUCTIBLE,
>     CPTK_IS_NOTHROW_ASSIGNABLE,
> -  CPTK_IS_NOTHROW_CONSTRUCTIBLE
> +  CPTK_IS_NOTHROW_CONSTRUCTIBLE,
> +  CPTK_REF_CONSTRUCTS_FROM_TEMPORARY,
> +  CPTK_REF_CONVERTS_FROM_TEMPORARY
>   };
>   
>   /* The types that we are processing.  */
> @@ -6520,7 +6522,7 @@ extern bool sufficient_parms_p			(const_tree);
>   extern tree type_decays_to			(tree);
>   extern tree extract_call_expr			(tree);
>   extern tree build_trivial_dtor_call		(tree, bool = false);
> -extern bool ref_conv_binds_directly_p		(tree, tree);
> +extern bool ref_conv_binds_to_temporary_p	(tree, tree, bool = false);
>   extern tree build_user_type_conversion		(tree, tree, int,
>   						 tsubst_flags_t);
>   extern tree build_new_function_call		(tree, vec<tree, va_gc> **,
> @@ -7105,6 +7107,7 @@ extern tree forward_parm			(tree);
>   extern bool is_trivially_xible			(enum tree_code, tree, tree);
>   extern bool is_nothrow_xible			(enum tree_code, tree, tree);
>   extern bool is_xible				(enum tree_code, tree, tree);
> +extern bool reference_from_temporary		(tree, tree, bool);
>   extern tree get_defaulted_eh_spec		(tree, tsubst_flags_t = tf_warning_or_error);
>   extern bool maybe_explain_implicit_delete	(tree);
>   extern void explain_implicit_non_constexpr	(tree);
> diff --git a/gcc/cp/cxx-pretty-print.cc b/gcc/cp/cxx-pretty-print.cc
> index 7e4db2e413b..44590830a61 100644
> --- a/gcc/cp/cxx-pretty-print.cc
> +++ b/gcc/cp/cxx-pretty-print.cc
> @@ -2696,6 +2696,12 @@ pp_cxx_trait_expression (cxx_pretty_printer *pp, tree t)
>       case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
>         pp_cxx_ws_string (pp, "__is_nothrow_constructible");
>         break;
> +    case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
> +      pp_cxx_ws_string (pp, "__reference_constructs_from_temporary");
> +      break;
> +    case CPTK_REF_CONVERTS_FROM_TEMPORARY:
> +      pp_cxx_ws_string (pp, "__reference_converts_from_temporary");
> +      break;
>   
>       default:
>         gcc_unreachable ();
> diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
> index 0dffd648b0b..dd9715b6725 100644
> --- a/gcc/cp/method.cc
> +++ b/gcc/cp/method.cc
> @@ -2211,6 +2211,34 @@ is_xible (enum tree_code code, tree to, tree from)
>     return !!expr;
>   }
>   
> +/* Return true iff conjunction_v<is_reference<T>, is_constructible<T, U>> is
> +   true, and the initialization
> +     T t(VAL<U>); // DIRECT_INIT_P
> +   or
> +     T t = VAL<U>; // !DIRECT_INIT_P
> +   binds t to a temporary object whose lifetime is extended.
> +   VAL<T> is defined in [meta.unary.prop]:
> +   -- If T is a reference or function type, VAL<T> is an expression with the
> +   same type and value category as declval<T>().
> +   -- Otherwise, VAL<T> is a prvalue that initially has type T.   */
> +
> +bool
> +reference_from_temporary (tree to, tree from, bool direct_init_p)
> +{
> +  /* Check is_reference<T>.  */
> +  if (!TYPE_REF_P (to))
> +    return false;
> +  /* Check is_constructible<T, U>.
> +     ??? This check doesn't seem to be necessary; if T isn't constructible
> +     from U, we won't be able to create a conversion.  */
> +  if (!is_xible (INIT_EXPR, to, build_tree_list (NULL_TREE, from)))
> +    return false;

I agree with the comment, did you try leaving this out?  If it stays I'd 
think it needs to consider direct_init_p.

> +  tree val = build_stub_object (from);
> +  if (!TYPE_REF_P (from) && TREE_CODE (from) != FUNCTION_TYPE)
> +    val = CLASS_TYPE_P (from) ? force_rvalue (val, tf_none) : rvalue (val);
> +  return ref_conv_binds_to_temporary_p (to, val, direct_init_p);
> +}
> +
>   /* Categorize various special_function_kinds.  */
>   #define SFK_CTOR_P(sfk) \
>     ((sfk) >= sfk_constructor && (sfk) <= sfk_move_constructor)
> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
> index bf9ea3685f8..edee94bda13 100644
> --- a/gcc/cp/parser.cc
> +++ b/gcc/cp/parser.cc
> @@ -5917,6 +5917,8 @@ cp_parser_primary_expression (cp_parser *parser,
>   	case RID_IS_CONSTRUCTIBLE:
>   	case RID_IS_NOTHROW_ASSIGNABLE:
>   	case RID_IS_NOTHROW_CONSTRUCTIBLE:
> +	case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
> +	case RID_REF_CONVERTS_FROM_TEMPORARY:
>   	  return cp_parser_trait_expr (parser, token->keyword);
>   
>   	// C++ concepts
> @@ -10988,6 +10990,14 @@ cp_parser_trait_expr (cp_parser* parser, enum rid keyword)
>         kind = CPTK_IS_NOTHROW_CONSTRUCTIBLE;
>         variadic = true;
>         break;
> +    case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
> +      kind = CPTK_REF_CONSTRUCTS_FROM_TEMPORARY;
> +      binary = true;
> +      break;
> +    case RID_REF_CONVERTS_FROM_TEMPORARY:
> +      kind = CPTK_REF_CONVERTS_FROM_TEMPORARY;
> +      binary = true;
> +      break;
>       default:
>         gcc_unreachable ();
>       }
> @@ -13811,7 +13821,7 @@ warn_for_range_copy (tree decl, tree expr)
>   
>     if (TYPE_REF_P (type))
>       {
> -      if (glvalue_p (expr) && !ref_conv_binds_directly_p (type, expr))
> +      if (glvalue_p (expr) && ref_conv_binds_to_temporary_p (type, expr))
>   	{
>   	  auto_diagnostic_group d;
>   	  if (warning_at (loc, OPT_Wrange_loop_construct,
> @@ -13842,7 +13852,7 @@ warn_for_range_copy (tree decl, tree expr)
>     tree rtype = cp_build_reference_type (type, /*rval*/false);
>     /* If we could initialize the reference directly, it wouldn't involve any
>        copies.  */
> -  if (!ref_conv_binds_directly_p (rtype, expr))
> +  if (ref_conv_binds_to_temporary_p (rtype, expr))
>       return;

I think this case wants the old handling of invalid conversions you 
mentioned in your intro; we don't want to suggest changing to a 
reference if that's ill-formed.

In passing we might change the comment to "If we can initialize a 
reference directly, suggest that to avoid the copy." and move it above 
the rtype declaration.

>     auto_diagnostic_group d;
> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> index 2344b5eea00..396a9f6b44e 100644
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -12007,6 +12007,12 @@ trait_expr_value (cp_trait_kind kind, tree type1, tree type2)
>       case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
>         return is_nothrow_xible (INIT_EXPR, type1, type2);
>   
> +    case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
> +      return reference_from_temporary (type1, type2, /*direct_init=*/true);
> +
> +    case CPTK_REF_CONVERTS_FROM_TEMPORARY:
> +      return reference_from_temporary (type1, type2, /*direct_init=*/false);
> +
>       default:
>         gcc_unreachable ();
>         return false;
> @@ -12088,6 +12094,8 @@ finish_trait_expr (location_t loc, cp_trait_kind kind, tree type1, tree type2)
>       case CPTK_IS_TRIVIALLY_CONSTRUCTIBLE:
>       case CPTK_IS_NOTHROW_ASSIGNABLE:
>       case CPTK_IS_NOTHROW_CONSTRUCTIBLE:
> +    case CPTK_REF_CONSTRUCTS_FROM_TEMPORARY:
> +    case CPTK_REF_CONVERTS_FROM_TEMPORARY:
>         if (!check_trait_type (type1)
>   	  || !check_trait_type (type2))
>   	return error_mark_node;
> diff --git a/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
> new file mode 100644
> index 00000000000..76de905a35d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/reference_constructs_from_temporary1.C
> @@ -0,0 +1,214 @@
> +// P2255R2
> +// PR c++/104477
> +// { dg-do compile { target c++11 } }
> +
> +#define SA(X) static_assert((X),#X)
> +
> +struct A { A(); };
> +struct B { operator A(); };
> +struct C { explicit C(); };
> +struct D { explicit operator A(); };
> +struct E { explicit operator A&(); };
> +struct F { explicit operator A&&(); };
> +// Could use a class template with explicit(bool), but then this would need
> +// C++20.
> +struct G {
> +  operator int();
> +  explicit operator int&&();
> +};
> +struct G2 {
> +  explicit operator int();
> +  operator int&&();
> +};
> +struct H {
> +  operator int();
> +  explicit operator int&();
> +};
> +struct H2 {
> +  explicit operator int();
> +  operator int&();
> +};
> +
> +struct Base { };
> +struct Der : Base { };
> +
> +template<typename T, typename RT>
> +struct morph {
> +  mutable T val{};
> +  operator RT() const { return static_cast<RT>(val); }
> +};
> +
> +template<typename T> using id = T;
> +
> +// Built-in types.
> +SA(!__reference_constructs_from_temporary(int, int));
> +SA(!__reference_constructs_from_temporary(int&, void));
> +SA(!__reference_constructs_from_temporary(int&, const void));
> +SA(!__reference_constructs_from_temporary(int&, volatile void));
> +SA(!__reference_constructs_from_temporary(int&, const volatile void));
> +SA(!__reference_constructs_from_temporary(void, void));
> +SA(!__reference_constructs_from_temporary(void, int));
> +SA(!__reference_constructs_from_temporary(int&, int));
> +SA(!__reference_constructs_from_temporary(int&, int&));
> +SA(!__reference_constructs_from_temporary(int&, int&&));
> +SA(!__reference_constructs_from_temporary(int&, long));
> +// non-const lvalue reference to type 'int' cannot bind to a value of unrelated type 'long'
> +SA(!__reference_constructs_from_temporary(int&, long&));
> +SA(!__reference_constructs_from_temporary(int&, long&&));
> +SA( __reference_constructs_from_temporary(const int&, int));
> +SA(!__reference_constructs_from_temporary(const int&, int&));
> +SA(!__reference_constructs_from_temporary(const int&, const int&));
> +SA(!__reference_constructs_from_temporary(const int&, int&&));
> +SA( __reference_constructs_from_temporary(const int&, long));
> +SA( __reference_constructs_from_temporary(const int&, long&));
> +SA( __reference_constructs_from_temporary(const int&, long&&));
> +SA( __reference_constructs_from_temporary(int&&, int));
> +SA(!__reference_constructs_from_temporary(int&&, int&));
> +SA(!__reference_constructs_from_temporary(int&&, int&&));
> +SA( __reference_constructs_from_temporary(int&&, long));
> +SA( __reference_constructs_from_temporary(int&&, long&));
> +SA( __reference_constructs_from_temporary(int&&, long&&));
> +SA(!__reference_constructs_from_temporary(unsigned int&, double));
> +SA(!__reference_constructs_from_temporary(volatile int&, int));
> +SA(!__reference_constructs_from_temporary(const volatile int&, int));
> +SA(!__reference_constructs_from_temporary(volatile int&, int&));
> +SA(!__reference_constructs_from_temporary(const volatile int&, int&));
> +SA(!__reference_constructs_from_temporary(volatile int&, int&&));
> +SA(!__reference_constructs_from_temporary(const volatile int&, int&&));
> +
> +// Classes.
> +SA(!__reference_constructs_from_temporary(A, A));
> +// A& r(A{}); doesn't construct.
> +SA(!__reference_constructs_from_temporary(A&, A));
> +SA(!__reference_constructs_from_temporary(A&, A&));
> +SA(!__reference_constructs_from_temporary(A&, A&&));
> +// Here we get const struct A & r = (const struct A &) &D.2414;
> +SA( __reference_constructs_from_temporary(const A&, A));
> +SA(!__reference_constructs_from_temporary(const A&, A&));
> +SA(!__reference_constructs_from_temporary(const A&, const A&));
> +SA(!__reference_constructs_from_temporary(const A&, A&&));
> +// Here we get struct A & r = (struct A &) &D.2439;
> +SA( __reference_constructs_from_temporary(A&&, A));
> +SA(!__reference_constructs_from_temporary(A&&, A&));
> +SA(!__reference_constructs_from_temporary(A&&, const A&));
> +SA(!__reference_constructs_from_temporary(A&&, A&&));
> +
> +SA(!__reference_constructs_from_temporary(A, B));
> +SA(!__reference_constructs_from_temporary(A&, B));
> +SA(!__reference_constructs_from_temporary(A&, B&));
> +SA(!__reference_constructs_from_temporary(A&, const B&));
> +SA(!__reference_constructs_from_temporary(A&, B&&));
> +SA( __reference_constructs_from_temporary(const A&, B));
> +SA( __reference_constructs_from_temporary(const A&, B&));
> +// Doesn't construct, so it's false.
> +SA(!__reference_constructs_from_temporary(const A&, const B&));
> +SA( __reference_constructs_from_temporary(const A&, B&&));
> +SA( __reference_constructs_from_temporary(A&&, B));
> +SA( __reference_constructs_from_temporary(A&&, B&));
> +SA(!__reference_constructs_from_temporary(A&&, const B&));
> +SA( __reference_constructs_from_temporary(A&&, B&&));
> +
> +SA(!__reference_constructs_from_temporary(const A&, C));
> +SA(!__reference_constructs_from_temporary(const A&, C&));
> +
> +// Doesn't construct, so it's false.
> +SA(!__reference_constructs_from_temporary(int&, morph<int, int>));
> +// Here we get
> +//   const int & r2 = D.2580 = morph<int, int>::operator int
> +//     (&TARGET_EXPR <D.2578, {.val=0}>); (const int &) &D.2580;
> +SA( __reference_constructs_from_temporary(const int&, morph<int, int>));
> +SA(!__reference_constructs_from_temporary(int&, morph<int, int&>));
> +SA(!__reference_constructs_from_temporary(int&, morph<int, const int&>));
> +SA(!__reference_constructs_from_temporary(int&, morph<int, int&&>));
> +SA( __reference_constructs_from_temporary(const int&, morph<long, long&>));
> +
> +// These are like const C& c(cref); so the explicit ctor C isn't a problem
> +// even in copy-init context.  const C& r = {}; would be a different story.
> +SA(!__reference_constructs_from_temporary(C, C));
> +SA(!__reference_constructs_from_temporary(C&, C));
> +SA(!__reference_constructs_from_temporary(C&, C&));
> +SA(!__reference_constructs_from_temporary(C&, C&&));
> +SA( __reference_constructs_from_temporary(const C&, C));
> +SA(!__reference_constructs_from_temporary(const C&, C&));
> +SA(!__reference_constructs_from_temporary(const C&, const C&));
> +SA(!__reference_constructs_from_temporary(const C&, C&&));
> +SA( __reference_constructs_from_temporary(C&&, C));
> +SA(!__reference_constructs_from_temporary(C&&, C&));
> +SA(!__reference_constructs_from_temporary(C&&, const C&));
> +SA(!__reference_constructs_from_temporary(C&&, C&&));
> +
> +// These are all false ultimately because of CWG 2267, which we implement.
> +SA(!__reference_constructs_from_temporary(A, D));
> +SA(!__reference_constructs_from_temporary(A&, D));
> +SA(!__reference_constructs_from_temporary(A&, D&));
> +SA(!__reference_constructs_from_temporary(A&, const D&));
> +SA(!__reference_constructs_from_temporary(A&, D&&));
> +SA(!__reference_constructs_from_temporary(const A&, D));
> +SA(!__reference_constructs_from_temporary(const A&, D&));
> +SA(!__reference_constructs_from_temporary(const A&, const D&));
> +SA(!__reference_constructs_from_temporary(const A&, D&&));
> +SA(!__reference_constructs_from_temporary(A&&, D));
> +SA(!__reference_constructs_from_temporary(A&&, D&));
> +SA(!__reference_constructs_from_temporary(A&&, const D&));
> +SA(!__reference_constructs_from_temporary(A&&, D&&));
> +
> +SA(!__reference_constructs_from_temporary(A, E));
> +/* A& a1(E{}); compiles, but A& a2 = E{}; doesn't.
> +   With the former, we get A& a = E::operator A& (&TARGET_EXPR <D.2715, {}>)
> +   so we're not binding the reference to a temporary, although there is
> +   a temporary involved.  So the result is false in both copy- and direct-
> +   init, albeit for different reasons!  */
> +SA(!__reference_constructs_from_temporary(A&, E));
> +// A& a = E::operator A& ((struct E *) r)); copy-init doesn't compile.
> +SA(!__reference_constructs_from_temporary(A&, E&));
> +SA(!__reference_constructs_from_temporary(A&, const E&));
> +SA(!__reference_constructs_from_temporary(A&, E&&));
> +// direct-init:
> +// const A& a = (const struct A &) E::operator A& (&TARGET_EXPR <D.2720, {}>)
> +SA(!__reference_constructs_from_temporary(const A&, E));
> +SA(!__reference_constructs_from_temporary(const A&, E&));
> +SA(!__reference_constructs_from_temporary(const A&, const E&));
> +SA(!__reference_constructs_from_temporary(const A&, E&&));
> +SA(!__reference_constructs_from_temporary(A&&, E));
> +SA(!__reference_constructs_from_temporary(A&&, E&));
> +SA(!__reference_constructs_from_temporary(A&&, const E&));
> +SA(!__reference_constructs_from_temporary(A&&, E&&));
> +
> +SA(!__reference_constructs_from_temporary(A, F));
> +// A& a1(F{}); and A& a2 = F{}; both invalid.
> +SA(!__reference_constructs_from_temporary(A&, F));
> +SA(!__reference_constructs_from_temporary(A&, F&));
> +SA(!__reference_constructs_from_temporary(A&, const F&));
> +SA(!__reference_constructs_from_temporary(A&, F&&));
> +SA(!__reference_constructs_from_temporary(const A&, F));
> +SA(!__reference_constructs_from_temporary(const A&, F&));
> +SA(!__reference_constructs_from_temporary(const A&, const F&));
> +SA(!__reference_constructs_from_temporary(const A&, F&&));
> +SA(!__reference_constructs_from_temporary(A&&, F));
> +SA(!__reference_constructs_from_temporary(A&&, F&));
> +SA(!__reference_constructs_from_temporary(A&&, const F&));
> +SA(!__reference_constructs_from_temporary(A&&, F&&));
> +
> +/* This is where _converts_ and _constructs_ will differ:
> +   in direct-init we use G::operator int&& (no temporary),
> +   but in copy-init we use G::operator int, where a temporary is created
> +   to be bound to int&&.  */
> +SA(!__reference_constructs_from_temporary(int&&, G));
> +// Similar to the previous one.
> +SA(!__reference_constructs_from_temporary(const int&, H));
> +/* And here I've switched the explicit-ness.  In both copy- and direct-init
> +   we call operator int&, so no temporary.  */
> +SA(!__reference_constructs_from_temporary(int&&, G2));
> +SA(!__reference_constructs_from_temporary(const int&, H2));
> +
> +SA(!__reference_constructs_from_temporary(const Base&, Der));
> +
> +// This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
> +SA(!__reference_constructs_from_temporary(int&&, id<int[3]>));
> +
> +// Arrays.
> +SA(!__reference_constructs_from_temporary(int, int[]));
> +SA(!__reference_constructs_from_temporary(int[], int[]));
> +SA(!__reference_constructs_from_temporary(int&, int[]));
> +SA(!__reference_constructs_from_temporary(int&&, int[]));
> +SA(!__reference_constructs_from_temporary(const int&, int[]));
> diff --git a/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
> new file mode 100644
> index 00000000000..90196c38742
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/ext/reference_converts_from_temporary1.C
> @@ -0,0 +1,214 @@
> +// P2255R2
> +// PR c++/104477
> +// { dg-do compile { target c++11 } }
> +
> +#define SA(X) static_assert((X),#X)
> +
> +struct A { A(); };
> +struct B { operator A(); };
> +struct C { explicit C(); };
> +struct D { explicit operator A(); };
> +struct E { explicit operator A&(); };
> +struct F { explicit operator A&&(); };
> +// Could use a class template with explicit(bool), but then this would need
> +// C++20.
> +struct G {
> +  operator int();
> +  explicit operator int&&();
> +};
> +struct G2 {
> +  explicit operator int();
> +  operator int&&();
> +};
> +struct H {
> +  operator int();
> +  explicit operator int&();
> +};
> +struct H2 {
> +  explicit operator int();
> +  operator int&();
> +};
> +
> +struct Base { };
> +struct Der : Base { };
> +
> +template<typename T, typename RT>
> +struct morph {
> +  mutable T val{};
> +  operator RT() const { return static_cast<RT>(val); }
> +};
> +
> +template<typename T> using id = T;
> +
> +// Built-in types.
> +SA(!__reference_converts_from_temporary(int, int));
> +SA(!__reference_converts_from_temporary(int&, void));
> +SA(!__reference_converts_from_temporary(int&, const void));
> +SA(!__reference_converts_from_temporary(int&, volatile void));
> +SA(!__reference_converts_from_temporary(int&, const volatile void));
> +SA(!__reference_converts_from_temporary(void, void));
> +SA(!__reference_converts_from_temporary(void, int));
> +SA(!__reference_converts_from_temporary(int&, int));
> +SA(!__reference_converts_from_temporary(int&, int&));
> +SA(!__reference_converts_from_temporary(int&, int&&));
> +SA(!__reference_converts_from_temporary(int&, long));
> +// non-const lvalue reference to type 'int' cannot bind to a value of unrelated type 'long'
> +SA(!__reference_converts_from_temporary(int&, long&));
> +SA(!__reference_converts_from_temporary(int&, long&&));
> +SA( __reference_converts_from_temporary(const int&, int));
> +SA(!__reference_converts_from_temporary(const int&, int&));
> +SA(!__reference_converts_from_temporary(const int&, const int&));
> +SA(!__reference_converts_from_temporary(const int&, int&&));
> +SA( __reference_converts_from_temporary(const int&, long));
> +SA( __reference_converts_from_temporary(const int&, long&));
> +SA( __reference_converts_from_temporary(const int&, long&&));
> +SA( __reference_converts_from_temporary(int&&, int));
> +SA(!__reference_converts_from_temporary(int&&, int&));
> +SA(!__reference_converts_from_temporary(int&&, int&&));
> +SA( __reference_converts_from_temporary(int&&, long));
> +SA( __reference_converts_from_temporary(int&&, long&));
> +SA( __reference_converts_from_temporary(int&&, long&&));
> +SA(!__reference_converts_from_temporary(unsigned int&, double));
> +SA(!__reference_converts_from_temporary(volatile int&, int));
> +SA(!__reference_converts_from_temporary(const volatile int&, int));
> +SA(!__reference_converts_from_temporary(volatile int&, int&));
> +SA(!__reference_converts_from_temporary(const volatile int&, int&));
> +SA(!__reference_converts_from_temporary(volatile int&, int&&));
> +SA(!__reference_converts_from_temporary(const volatile int&, int&&));
> +
> +// Classes.
> +SA(!__reference_converts_from_temporary(A, A));
> +// A& r(A{}); doesn't construct.
> +SA(!__reference_converts_from_temporary(A&, A));
> +SA(!__reference_converts_from_temporary(A&, A&));
> +SA(!__reference_converts_from_temporary(A&, A&&));
> +// Here we get const struct A & r = (const struct A &) &D.2414;
> +SA( __reference_converts_from_temporary(const A&, A));
> +SA(!__reference_converts_from_temporary(const A&, A&));
> +SA(!__reference_converts_from_temporary(const A&, const A&));
> +SA(!__reference_converts_from_temporary(const A&, A&&));
> +// Here we get struct A & r = (struct A &) &D.2439;
> +SA( __reference_converts_from_temporary(A&&, A));
> +SA(!__reference_converts_from_temporary(A&&, A&));
> +SA(!__reference_converts_from_temporary(A&&, const A&));
> +SA(!__reference_converts_from_temporary(A&&, A&&));
> +
> +SA(!__reference_converts_from_temporary(A, B));
> +SA(!__reference_converts_from_temporary(A&, B));
> +SA(!__reference_converts_from_temporary(A&, B&));
> +SA(!__reference_converts_from_temporary(A&, const B&));
> +SA(!__reference_converts_from_temporary(A&, B&&));
> +SA( __reference_converts_from_temporary(const A&, B));
> +SA( __reference_converts_from_temporary(const A&, B&));
> +// Doesn't construct, so it's false.
> +SA(!__reference_converts_from_temporary(const A&, const B&));
> +SA( __reference_converts_from_temporary(const A&, B&&));
> +SA( __reference_converts_from_temporary(A&&, B));
> +SA( __reference_converts_from_temporary(A&&, B&));
> +SA(!__reference_converts_from_temporary(A&&, const B&));
> +SA( __reference_converts_from_temporary(A&&, B&&));
> +
> +SA(!__reference_converts_from_temporary(const A&, C));
> +SA(!__reference_converts_from_temporary(const A&, C&));
> +
> +// Doesn't construct, so it's false.
> +SA(!__reference_converts_from_temporary(int&, morph<int, int>));
> +// Here we get
> +//   const int & r2 = D.2580 = morph<int, int>::operator int
> +//     (&TARGET_EXPR <D.2578, {.val=0}>); (const int &) &D.2580;
> +SA( __reference_converts_from_temporary(const int&, morph<int, int>));
> +SA(!__reference_converts_from_temporary(int&, morph<int, int&>));
> +SA(!__reference_converts_from_temporary(int&, morph<int, const int&>));
> +SA(!__reference_converts_from_temporary(int&, morph<int, int&&>));
> +SA( __reference_converts_from_temporary(const int&, morph<long, long&>));
> +
> +// These are like const C& c(cref); so the explicit ctor C isn't a problem
> +// even in copy-init context.  const C& r = {}; would be a different story.
> +SA(!__reference_converts_from_temporary(C, C));
> +SA(!__reference_converts_from_temporary(C&, C));
> +SA(!__reference_converts_from_temporary(C&, C&));
> +SA(!__reference_converts_from_temporary(C&, C&&));
> +SA( __reference_converts_from_temporary(const C&, C));
> +SA(!__reference_converts_from_temporary(const C&, C&));
> +SA(!__reference_converts_from_temporary(const C&, const C&));
> +SA(!__reference_converts_from_temporary(const C&, C&&));
> +SA( __reference_converts_from_temporary(C&&, C));
> +SA(!__reference_converts_from_temporary(C&&, C&));
> +SA(!__reference_converts_from_temporary(C&&, const C&));
> +SA(!__reference_converts_from_temporary(C&&, C&&));
> +
> +// These are all false ultimately because of CWG 2267, which we implement.
> +SA(!__reference_converts_from_temporary(A, D));
> +SA(!__reference_converts_from_temporary(A&, D));
> +SA(!__reference_converts_from_temporary(A&, D&));
> +SA(!__reference_converts_from_temporary(A&, const D&));
> +SA(!__reference_converts_from_temporary(A&, D&&));
> +SA(!__reference_converts_from_temporary(const A&, D));
> +SA(!__reference_converts_from_temporary(const A&, D&));
> +SA(!__reference_converts_from_temporary(const A&, const D&));
> +SA(!__reference_converts_from_temporary(const A&, D&&));
> +SA(!__reference_converts_from_temporary(A&&, D));
> +SA(!__reference_converts_from_temporary(A&&, D&));
> +SA(!__reference_converts_from_temporary(A&&, const D&));
> +SA(!__reference_converts_from_temporary(A&&, D&&));
> +
> +SA(!__reference_converts_from_temporary(A, E));
> +/* A& a1(E{}); compiles, but A& a2 = E{}; doesn't.
> +   With the former, we get A& a = E::operator A& (&TARGET_EXPR <D.2715, {}>)
> +   so we're not binding the reference to a temporary, although there is
> +   a temporary involved.  So the result is false in both copy- and direct-
> +   init, albeit for different reasons!  */
> +SA(!__reference_converts_from_temporary(A&, E));
> +// A& a = E::operator A& ((struct E *) r)); copy-init doesn't compile.
> +SA(!__reference_converts_from_temporary(A&, E&));
> +SA(!__reference_converts_from_temporary(A&, const E&));
> +SA(!__reference_converts_from_temporary(A&, E&&));
> +// direct-init:
> +// const A& a = (const struct A &) E::operator A& (&TARGET_EXPR <D.2720, {}>)
> +SA(!__reference_converts_from_temporary(const A&, E));
> +SA(!__reference_converts_from_temporary(const A&, E&));
> +SA(!__reference_converts_from_temporary(const A&, const E&));
> +SA(!__reference_converts_from_temporary(const A&, E&&));
> +SA(!__reference_converts_from_temporary(A&&, E));
> +SA(!__reference_converts_from_temporary(A&&, E&));
> +SA(!__reference_converts_from_temporary(A&&, const E&));
> +SA(!__reference_converts_from_temporary(A&&, E&&));
> +
> +SA(!__reference_converts_from_temporary(A, F));
> +// A& a1(F{}); and A& a2 = F{}; both invalid.
> +SA(!__reference_converts_from_temporary(A&, F));
> +SA(!__reference_converts_from_temporary(A&, F&));
> +SA(!__reference_converts_from_temporary(A&, const F&));
> +SA(!__reference_converts_from_temporary(A&, F&&));
> +SA(!__reference_converts_from_temporary(const A&, F));
> +SA(!__reference_converts_from_temporary(const A&, F&));
> +SA(!__reference_converts_from_temporary(const A&, const F&));
> +SA(!__reference_converts_from_temporary(const A&, F&&));
> +SA(!__reference_converts_from_temporary(A&&, F));
> +SA(!__reference_converts_from_temporary(A&&, F&));
> +SA(!__reference_converts_from_temporary(A&&, const F&));
> +SA(!__reference_converts_from_temporary(A&&, F&&));
> +
> +/* This is where _converts_ and _constructs_ will differ:
> +   in direct-init we use G::operator int&& (no temporary),
> +   but in copy-init we use G::operator int, where a temporary is created
> +   to be bound to int&&.  */
> +SA( __reference_converts_from_temporary(int&&, G));
> +// Similar to the previous one.
> +SA( __reference_converts_from_temporary(const int&, H));
> +/* And here I've switched the explicit-ness.  In both copy- and direct-init
> +   we call operator int&, so no temporary.  */
> +SA(!__reference_converts_from_temporary(int&&, G2));
> +SA(!__reference_converts_from_temporary(const int&, H2));
> +
> +SA(!__reference_converts_from_temporary(const Base&, Der));
> +
> +// This fails because std::is_constructible_v<int&&, id<int[3]>> is false.
> +SA(!__reference_converts_from_temporary(int&&, id<int[3]>));
> +
> +// Arrays.
> +SA(!__reference_converts_from_temporary(int, int[]));
> +SA(!__reference_converts_from_temporary(int[], int[]));
> +SA(!__reference_converts_from_temporary(int&, int[]));
> +SA(!__reference_converts_from_temporary(int&&, int[]));
> +SA(!__reference_converts_from_temporary(const int&, int[]));
> diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
> index 2572d8edd69..bbeb049e1f9 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -3508,6 +3508,45 @@ template<typename _Ret, typename _Fn, typename... _Args>
>     template<typename _Tp>
>       inline constexpr bool is_scoped_enum_v = is_scoped_enum<_Tp>::value;
>   
> +#define __cpp_lib_reference_from_temporary 202202L
> +
> +  /// True if _Tp is a reference type, a _Up value can be bound to _Tp in
> +  /// direct-initialization, and a temporary object would be bound to
> +  /// the reference, false otherwise.
> +  /// @since C++23
> +  template<typename _Tp, typename _Up>
> +    struct reference_constructs_from_temporary
> +    : public __bool_constant<__reference_constructs_from_temporary(_Tp, _Up)>
> +    {
> +      static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{})
> +		    && std::__is_complete_or_unbounded(__type_identity<_Up>{}),
> +	"template argument must be a complete class or an unbounded array");
> +    };
> +
> +  /// True if _Tp is a reference type, a _Up value can be bound to _Tp in
> +  /// copy-initialization, and a temporary object would be bound to
> +  /// the reference, false otherwise.
> +  /// @since C++23
> +  template<typename _Tp, typename _Up>
> +    struct reference_converts_from_temporary
> +    : public __bool_constant<__reference_converts_from_temporary(_Tp, _Up)>
> +    {
> +      static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{})
> +		    && std::__is_complete_or_unbounded(__type_identity<_Up>{}),
> +	"template argument must be a complete class or an unbounded array");
> +    };
> +
> +  /// @ingroup variable_templates
> +  /// @since C++23
> +  template<typename _Tp, typename _Up>
> +    inline constexpr bool reference_constructs_from_temporary_v
> +      = reference_constructs_from_temporary<_Tp, _Up>::value;
> +
> +  /// @ingroup variable_templates
> +  /// @since C++23
> +  template<typename _Tp, typename _Up>
> +    inline constexpr bool reference_converts_from_temporary_v
> +      = reference_converts_from_temporary<_Tp, _Up>::value;
>   #endif // C++23
>   
>   #if _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
> diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
> index 22280e1a349..5edca2f3007 100644
> --- a/libstdc++-v3/include/std/version
> +++ b/libstdc++-v3/include/std/version
> @@ -300,10 +300,11 @@
>   #endif
>   
>   #if __cplusplus > 202002L
> -// c++2b
> +// c++23
>   #define __cpp_lib_byteswap 202110L
>   #define __cpp_lib_constexpr_typeinfo 202106L
>   #define __cpp_lib_is_scoped_enum 202011L
> +#define __cpp_lib_reference_from_temporary 202202L
>   
>   #if _GLIBCXX_HOSTED
>   #define __cpp_lib_adaptor_iterator_pair_constructor 202106L
> @@ -335,7 +336,7 @@
>   #define __cpp_lib_to_underlying 202102L
>   #define __cpp_lib_unreachable 202202L
>   #endif
> -#endif // C++2b
> +#endif // C++23
>   #endif // C++20
>   #endif // C++17
>   #endif // C++14
> diff --git a/libstdc++-v3/testsuite/20_util/reference_from_temporary/value.cc b/libstdc++-v3/testsuite/20_util/reference_from_temporary/value.cc
> new file mode 100644
> index 00000000000..2f62e54d46d
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/reference_from_temporary/value.cc
> @@ -0,0 +1,110 @@
> +// Copyright (C) 2022 Free Software Foundation, Inc.
> +//
> +// This file is part of the GNU ISO C++ Library.  This library is free
> +// software; you can redistribute it and/or modify it under the
> +// terms of the GNU General Public License as published by the
> +// Free Software Foundation; either version 3, or (at your option)
> +// any later version.
> +
> +// This library is distributed in the hope that it will be useful,
> +// but WITHOUT ANY WARRANTY; without even the implied warranty of
> +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +// GNU General Public License for more details.
> +
> +// You should have received a copy of the GNU General Public License along
> +// with this library; see the file COPYING3.  If not see
> +// <http://www.gnu.org/licenses/>.
> +
> +// { dg-options "-std=gnu++23" }
> +// { dg-do compile { target c++23 } }
> +
> +#include <type_traits>
> +#include <testsuite_tr1.h>
> +
> +#ifndef __cpp_lib_reference_from_temporary
> +# error "Feature test macro for reference_from_temporary is missing in <version>"
> +#elif __cpp_lib_reference_from_temporary < 202202L
> +# error "Feature test macro for reference_from_temporary has wrong value in <version>"
> +#endif
> +
> +void test01()
> +{
> +  using std::reference_constructs_from_temporary;
> +  using std::reference_converts_from_temporary;
> +  using namespace __gnu_test;
> +
> +  struct A { A(); };
> +
> +  struct B {
> +    operator int();
> +    explicit operator int&&();
> +  };
> +
> +  struct C {
> +    operator int();
> +    explicit operator int&();
> +  };
> +
> +  static_assert(test_property<reference_constructs_from_temporary, int, int>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, void>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, const volatile void>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, void, void>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, int>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, int&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, int&&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, long>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, long&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, long&&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, int>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, int&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, int&&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, long>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, long&>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, long&&>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, int>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, int&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, int&&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, long>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, long&>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, long&&>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const A&, A>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const A&, A&&>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, A&&, A>(true), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&, int[]>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, int[]>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, int[]>(false), "");
> +
> +  static_assert(test_property<reference_converts_from_temporary, int, int>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, void>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, const volatile void>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, void, void>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, int>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, int&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, int&&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, long>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, long&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, long&&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, int>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, int&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, int&&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, long>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, long&>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, long&&>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, int>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, int&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, int&&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, long>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, long&>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, long&&>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, const A&, A>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, const A&, A&&>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, A&&, A>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&, int[]>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, int[]>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, int[]>(false), "");
> +
> +  static_assert(test_property<reference_constructs_from_temporary, int&&, B>(false), "");
> +  static_assert(test_property<reference_constructs_from_temporary, const int&, C>(false), "");
> +  static_assert(test_property<reference_converts_from_temporary, int&&, B>(true), "");
> +  static_assert(test_property<reference_converts_from_temporary, const int&, C>(true), "");
> +}
> diff --git a/libstdc++-v3/testsuite/20_util/reference_from_temporary/value2.cc b/libstdc++-v3/testsuite/20_util/reference_from_temporary/value2.cc
> new file mode 100644
> index 00000000000..65770754299
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/reference_from_temporary/value2.cc
> @@ -0,0 +1,28 @@
> +// Copyright (C) 2022 Free Software Foundation, Inc.
> +//
> +// This file is part of the GNU ISO C++ Library.  This library is free
> +// software; you can redistribute it and/or modify it under the
> +// terms of the GNU General Public License as published by the
> +// Free Software Foundation; either version 3, or (at your option)
> +// any later version.
> +
> +// This library is distributed in the hope that it will be useful,
> +// but WITHOUT ANY WARRANTY; without even the implied warranty of
> +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +// GNU General Public License for more details.
> +
> +// You should have received a copy of the GNU General Public License along
> +// with this library; see the file COPYING3.  If not see
> +// <http://www.gnu.org/licenses/>.
> +
> +// { dg-options "-std=gnu++23" }
> +// { dg-do compile { target c++23 } }
> +
> +#include <type_traits>
> +#include <string>
> +
> +void test01()
> +{
> +  static_assert(std::reference_converts_from_temporary_v<const std::string&, const char*>);
> +  static_assert(std::reference_constructs_from_temporary_v<const std::string&, const char*>);
> +}
> diff --git a/libstdc++-v3/testsuite/20_util/reference_from_temporary/version.cc b/libstdc++-v3/testsuite/20_util/reference_from_temporary/version.cc
> new file mode 100644
> index 00000000000..f56e7c0dabc
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/reference_from_temporary/version.cc
> @@ -0,0 +1,27 @@
> +// Copyright (C) 2022 Free Software Foundation, Inc.
> +//
> +// This file is part of the GNU ISO C++ Library.  This library is free
> +// software; you can redistribute it and/or modify it under the
> +// terms of the GNU General Public License as published by the
> +// Free Software Foundation; either version 3, or (at your option)
> +// any later version.
> +
> +// This library is distributed in the hope that it will be useful,
> +// but WITHOUT ANY WARRANTY; without even the implied warranty of
> +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +// GNU General Public License for more details.
> +
> +// You should have received a copy of the GNU General Public License along
> +// with this library; see the file COPYING3.  If not see
> +// <http://www.gnu.org/licenses/>.
> +
> +// { dg-options "-std=gnu++23" }
> +// { dg-do compile { target c++23 } }
> +
> +#include <version>
> +
> +#ifndef __cpp_lib_reference_from_temporary
> +# error "Feature test macro for reference_from_temporary is missing in <version>"
> +#elif __cpp_lib_reference_from_temporary < 202202L
> +# error "Feature test macro for reference_from_temporary has wrong value in <version>"
> +#endif
> diff --git a/libstdc++-v3/testsuite/20_util/variable_templates_for_traits.cc b/libstdc++-v3/testsuite/20_util/variable_templates_for_traits.cc
> index 9b3957f7d47..2b03ad7067d 100644
> --- a/libstdc++-v3/testsuite/20_util/variable_templates_for_traits.cc
> +++ b/libstdc++-v3/testsuite/20_util/variable_templates_for_traits.cc
> @@ -346,3 +346,17 @@ static_assert(disjunction_v<false_type, false_type,
>                 true_type>, "");
>   static_assert(!disjunction_v<false_type, false_type,
>                 false_type>, "");
> +#if __cpp_lib_reference_from_temporary >= 202202L
> +static_assert(std::reference_converts_from_temporary_v<int&&, int>
> +	      && std::reference_converts_from_temporary_v<const int&, int>
> +	      && !std::reference_converts_from_temporary_v<int&&, int&&>
> +	      && !std::reference_converts_from_temporary_v<const int&, int&&>
> +	      && std::reference_converts_from_temporary_v<int&&, long&&>
> +	      && std::reference_converts_from_temporary_v<int&&, long>, "");
> +static_assert(std::reference_constructs_from_temporary_v<int&&, int>
> +	      && std::reference_constructs_from_temporary_v<const int&, int>
> +	      && !std::reference_constructs_from_temporary_v<int&&, int&&>
> +	      && !std::reference_constructs_from_temporary_v<const int&, int&&>
> +	      && std::reference_constructs_from_temporary_v<int&&, long&&>
> +	      && std::reference_constructs_from_temporary_v<int&&, long>, "");
> +#endif
> 
> base-commit: 55bb77b50a5ec520f28978ac3fdac1983200e1f7