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Implement P0504R0 (Revisiting in-place tag types for any/optional/variant). * include/std/any (any(_ValueType&& __value)): Constrain the __is_in_place_type with the decayed type. (make_any): Adjust to use the new tag type. * include/std/utility (in_place_tag): Remove. (in_place_t): Turn into a non-reference tag type. (__in_place, __in_place_type, __in_place_index): Remove. (in_place): Turn into an inline variable of non-reference tag type. (in_place<_Tp>): Remove. (in_place_index<_Idx>): Remove. (in_place_type_t): New. (in_place_type): Turn into a variable template of non-reference type. (in_place_index_t): New. (in_place_index): Turn into a variable template of non-reference type. * include/std/variant (_Variant_storage(in_place_index_t<_Np>, _Args&&...)): Adjust to use the new tag type. (_Union(in_place_index_t<0>, _Args&&...)): Likewise. (_Union(in_place_index_t<_Np>, _Args&&...)): Likewise. (_Variant_base()): Likewise. (variant(_Tp&&)): Likewise. (variant(in_place_type_t<_Tp>, _Args&&...)): Likewise. (variant(in_place_type_t<_Tp>, initializer_list<_Up>, _Args&&...)): Likewise. (variant(in_place_index_t<_Np>, _Args&&...)): Likewise. (variant(in_place_index_t<_Np>, initializer_list<_Up>, _Args&&...)): Likewise (variant(allocator_arg_t, const _Alloc&)): Likewise. (variant(allocator_arg_t, const _Alloc&, _Tp&&)): Likewise. (variant(allocator_arg_t, const _Alloc&, in_place_type_t<_Tp>, _Args&&...)): Likewise. (variant(allocator_arg_t, const _Alloc&, in_place_type_t<_Tp>, initializer_list<_Up>, _Args&&...)): Likewise. (variant(allocator_arg_t, const _Alloc&, in_place_index_t<_Np>, _Args&&...)): Likewise. (variant(allocator_arg_t, const _Alloc&, in_place_index_t<_Np>, initializer_list<_Up>, _Args&&...)): Likewise. (emplace(_Args&&...)): Likewise. (emplace(initializer_list<_Up>, _Args&&...)): Likewise. * testsuite/20_util/any/cons/explicit.cc: Likewise. * testsuite/20_util/any/cons/in_place.cc: Likewise. * testsuite/20_util/any/requirements.cc: Add tests to check that any is not constructible from the new in_place_type_t of any value category. * testsuite/20_util/in_place/requirements.cc: Adjust to use the new tag type. * testsuite/20_util/variant/compile.cc: Likewise. * testsuite/20_util/variant/run.cc: Likewise. From-SVN: r242401