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

[Ville Voutilainen <ville.voutilainen@gmail.com>]

Well, is_xible is not is_xible_p because it doesn't need to be both is_*
and *_p. But xes_from_temporary is less obviously a question, so
xes_from_temporary_p would imho be a better name.

On Fri, Jul 15, 2022, 18:33 Marek Polacek via Libstdc++ <
libstdc++@gcc.gnu.org> wrote:

> On Thu, Jul 14, 2022 at 11:48:51PM -0400, Jason Merrill wrote:
> > On 7/14/22 13:43, Marek Polacek wrote:
> > > On Tue, Jul 12, 2022 at 04:15:00PM -0400, Jason Merrill wrote:
> > > > On 7/12/22 16:10, Jason Merrill wrote:
> > > > > 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.
> > > >
> > > > Hmm, and I suspect we get false positives when expr is a prvalue, so
> > > > initializing a non-reference variable of the same cv-unqualified type
> > > > involves no extra copy?
> > >
> > > I couldn't provoke a false positive here.  Note that
> ref_conv_binds_to_temporary_p
> > > always gets a reference so copy elision isn't applicable here.
> >
> > Ah, I see: if expr is a prvalue, that counts as binding a reference to a
> > temporary, so we (properly) don't think the reference is an improvement.
> >
> > The function name reference_from_temporary seems to me to suggest a tree
> > value rather than bool, maybe rename to ref_xes_from_temporary?
>
> I agree that that's a better name, so I've changed it.  I would've
> preferred
> ref_xes_from_temporary_p but is_xible et al don't have the _p.
>
> I'm going to push this, thanks!
>
> Tested x86_64-pc-linux-gnu, applying to trunk.
>
> -- >8 --
> 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 the conv_binds_ref_to_prvalue function.  The main function here is
> ref_xes_from_temporary.
> I've changed the return type of ref_conv_binds_directly to tristate,
> because
> previously the function didn't distinguish between an invalid conversion
> and
> one that binds to a prvalue.  Since it no longer returns a bool, I removed
> the _p suffix.
>
> The patch also adds the relevant class and variable templates to
> <type_traits>.
>
>         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_directly): ... this.  Add a new bool parameter.
> Change
>         the return type to tristate.
>         * constraint.cc (diagnose_trait_expr): Handle
>         CPTK_REF_CONSTRUCTS_FROM_TEMPORARY and
> CPTK_REF_CONVERTS_FROM_TEMPORARY.
>         * cp-tree.h: Include "tristate.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_directly): ... this.
>         (ref_xes_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 (ref_xes_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_directly.
>         * 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                                |  20 +-
>  gcc/cp/constraint.cc                          |   8 +
>  gcc/cp/cp-tree.h                              |   8 +-
>  gcc/cp/cxx-pretty-print.cc                    |   6 +
>  gcc/cp/method.cc                              |  25 ++
>  gcc/cp/parser.cc                              |  36 +--
>  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, 744 insertions(+), 24 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..191c68cdcfd 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -9109,21 +9109,27 @@ 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.  */
> +/* Return tristate::TS_TRUE if converting EXPR to a reference type TYPE
> does
> +   not involve creating a temporary.  Return tristate::TS_FALSE if
> converting
> +   EXPR to a reference type TYPE binds the reference to a temporary.  If
> the
> +   conversion is invalid or bad, return tristate::TS_UNKNOWN.
> 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)
> +tristate
> +ref_conv_binds_directly (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);
> +  tristate ret (tristate::TS_UNKNOWN);
> +  if (conv && !conv->bad_p)
> +    ret = tristate (!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 f2137eb7aa1..568318f0ba1 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -3697,6 +3697,14 @@ 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 bec98aa2ac3..cf51c39dc90 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -24,6 +24,7 @@ along with GCC; see the file COPYING3.  If not see
>  #include "tm.h"
>  #include "hard-reg-set.h"
>  #include "function.h"
> +#include "tristate.h"
>
>  /* In order for the format checking to accept the C++ front end
>     diagnostic framework extensions, you must include this file before
> @@ -1397,7 +1398,9 @@ 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 +6523,7 @@ 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 tristate ref_conv_binds_directly                (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 +7108,7 @@ 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 ref_xes_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..f2050f6e970 100644
> --- a/gcc/cp/method.cc
> +++ b/gcc/cp/method.cc
> @@ -2211,6 +2211,31 @@ 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
> +ref_xes_from_temporary (tree to, tree from, bool direct_init_p)
> +{
> +  /* Check is_reference<T>.  */
> +  if (!TYPE_REF_P (to))
> +    return false;
> +  /* We don't check is_constructible<T, U>: if T isn't constructible
> +     from U, we won't be able to create a conversion.  */
> +  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_directly (to, val, direct_init_p).is_false ();
> +}
> +
>  /* 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..4f67441eeb1 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_directly (type,
> expr).is_false ())
>         {
>           auto_diagnostic_group d;
>           if (warning_at (loc, OPT_Wrange_loop_construct,
> @@ -13839,20 +13849,20 @@ warn_for_range_copy (tree decl, tree expr)
>           && trivially_copyable_p (type)))
>      return;
>
> +  /* If we can initialize a reference directly, suggest that to avoid the
> +     copy.  */
>    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))
> -    return;
> -
> -  auto_diagnostic_group d;
> -  if (warning_at (loc, OPT_Wrange_loop_construct,
> -                 "loop variable %qD creates a copy from type %qT",
> -                 decl, type))
> +  if (ref_conv_binds_directly (rtype, expr).is_true ())
>      {
> -      gcc_rich_location richloc (loc);
> -      richloc.add_fixit_insert_before ("&");
> -      inform (&richloc, "use reference type to prevent copying");
> +      auto_diagnostic_group d;
> +      if (warning_at (loc, OPT_Wrange_loop_construct,
> +                     "loop variable %qD creates a copy from type %qT",
> +                     decl, type))
> +       {
> +         gcc_rich_location richloc (loc);
> +         richloc.add_fixit_insert_before ("&");
> +         inform (&richloc, "use reference type to prevent copying");
> +       }
>      }
>  }
>
> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> index 2344b5eea00..96037c21b85 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 ref_xes_from_temporary (type1, type2, /*direct_init=*/true);
> +
> +    case CPTK_REF_CONVERTS_FROM_TEMPORARY:
> +      return ref_xes_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 e5f58bc2e3f..b1a1deecf66 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -3505,6 +3505,45 @@ 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: 23dd41c480fa9f06c33c1e6090bbae53869f85af
> --
> 2.36.1
>
>