Implement P2255R2 Type Traits To Detect References Binding To Temporaries for Clang

stl/inc/memory

@@ -3475,6 +3475,11 @@ public:
     }
 
     _NODISCARD _CONSTEXPR23 add_lvalue_reference_t<_Ty> operator*() const noexcept(noexcept(*_STD declval<pointer>())) {
+#ifdef __cpp_lib_reference_from_temporary
+        static_assert(
+            !reference_converts_from_temporary_v<add_lvalue_reference_t<_Ty>, decltype(*_STD declval<pointer>())>,
+            "Can't return a temporary object from unique_ptr::operator*(). (N5014 [unique.ptr.single.observers]/1)");
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
         return *_Mypair._Myval2;
     }
 

stl/inc/type_traits

@@ -748,6 +748,47 @@ _EXPORT_STD template <class _Ty>
 _NO_SPECIALIZATIONS_OF_TYPE_TRAITS constexpr bool has_unique_object_representations_v =
     __has_unique_object_representations(_Ty);
 
+#ifdef __cpp_lib_reference_from_temporary
+#ifdef __clang__ // TRANSITION, LLVM-114344
+template <class _Ty>
+struct _Adjust_ref_binding_source {
+    using type = _Ty;
+};
+template <class _Ty>
+    requires is_scalar_v<_Ty> || is_void_v<_Ty>
+struct _Adjust_ref_binding_source<_Ty> {
+    using type = remove_cv_t<_Ty>;
+};
+template <class _Ty>
+    requires is_function_v<_Ty> && requires { typename void_t<_Ty&>; }
+struct _Adjust_ref_binding_source<_Ty> {
+    using type = _Ty&;
+};
+
+template <class _Ty>
+using _Adjust_ref_binding_source_t = _Adjust_ref_binding_source<_Ty>::type;
+#else // ^^^ workaround / no workaround vvv
+template <class _Ty>
+using _Adjust_ref_binding_source_t = _Ty;
+#endif // ^^^ no workaround ^^^
+
+_EXPORT_STD template <class _Ty, class _Uty>
+struct _NO_SPECIALIZATIONS_OF_TYPE_TRAITS reference_constructs_from_temporary
+    : bool_constant<__reference_constructs_from_temporary(_Ty, _Adjust_ref_binding_source_t<_Uty>)> {};
+
+_EXPORT_STD template <class _Ty, class _Uty>
+_NO_SPECIALIZATIONS_OF_TYPE_TRAITS constexpr bool reference_constructs_from_temporary_v =
+    __reference_constructs_from_temporary(_Ty, _Adjust_ref_binding_source_t<_Uty>);
+
+_EXPORT_STD template <class _Ty, class _Uty>
+struct _NO_SPECIALIZATIONS_OF_TYPE_TRAITS reference_converts_from_temporary
+    : bool_constant<__reference_converts_from_temporary(_Ty, _Adjust_ref_binding_source_t<_Uty>)> {};
+
+_EXPORT_STD template <class _Ty, class _Uty>
+_NO_SPECIALIZATIONS_OF_TYPE_TRAITS constexpr bool reference_converts_from_temporary_v =
+    __reference_converts_from_temporary(_Ty, _Adjust_ref_binding_source_t<_Uty>);
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
+
 #ifdef __EDG__ // TRANSITION, VSO-1690654
 template <class _Ty>
 struct _Is_aggregate_impl : bool_constant<__is_aggregate(_Ty)> {};
@@ -1823,9 +1864,17 @@ struct _NO_SPECIALIZATIONS_OF_TYPE_TRAITS is_nothrow_convertible
 template <class _From, class _To, class = void>
 struct _Invoke_convertible : false_type {};
 
+#ifdef __cpp_lib_reference_from_temporary
+template <class _To, class _From>
+using _Not_reference_converts_from_temporary = bool_constant<!reference_converts_from_temporary_v<_To, _From>>;
+#else // ^^^ defined(__cpp_lib_reference_from_temporary) / !defined(__cpp_lib_reference_from_temporary) vvv
+template <class _To, class _From>
+using _Not_reference_converts_from_temporary = true_type;
+#endif // ^^^ !defined(__cpp_lib_reference_from_temporary) ^^^
+
 template <class _From, class _To>
 struct _Invoke_convertible<_From, _To, void_t<decltype(_STD _Fake_copy_init<_To>(_STD _Returns_exactly<_From>()))>>
-    : true_type {};
+    : _Not_reference_converts_from_temporary<_To, _From> {};
 
 template <class _From, class _To>
 struct _Invoke_nothrow_convertible

stl/inc/utility

@@ -275,6 +275,14 @@ struct pair { // store a pair of values
             is_nothrow_constructible_v<_Ty1, _Other1> && is_nothrow_constructible_v<_Ty2, _Other2>) // strengthened
         : first(_STD forward<_Other1>(_Val1)), second(_STD forward<_Other2>(_Val2)) {
     }
+#ifdef __cpp_lib_reference_from_temporary
+    template <class _Other1 = _Ty1, class _Other2 = _Ty2,
+        enable_if_t<conjunction_v<is_constructible<_Ty1, _Other1>, is_constructible<_Ty2, _Other2>>, int> = 0>
+        requires reference_constructs_from_temporary_v<_Ty1, _Other1&&>
+                  || reference_constructs_from_temporary_v<_Ty2, _Other2&&>
+    explicit(!conjunction_v<is_convertible<_Other1, _Ty1>, is_convertible<_Other2, _Ty2>>)
+        pair(_Other1&&, _Other2&&) = delete;
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
 
     pair(const pair&) = default;
     pair(pair&&)      = default;
@@ -286,6 +294,14 @@ struct pair { // store a pair of values
         pair(pair<_Other1, _Other2>& _Right) noexcept(
             is_nothrow_constructible_v<_Ty1, _Other1&> && is_nothrow_constructible_v<_Ty2, _Other2&>) // strengthened
         : first(_Right.first), second(_Right.second) {}
+#ifdef __cpp_lib_reference_from_temporary
+    template <class _Other1, class _Other2>
+        requires is_constructible_v<_Ty1, _Other1&> && is_constructible_v<_Ty2, _Other2&>
+                  && (reference_constructs_from_temporary_v<_Ty1, _Other1&>
+                      || reference_constructs_from_temporary_v<_Ty2, _Other2&>)
+    explicit(!conjunction_v<is_convertible<_Other1&, _Ty1>, is_convertible<_Other2&, _Ty2>>)
+        pair(pair<_Other1, _Other2>&) = delete;
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
 #endif // _HAS_CXX23
 
     template <class _Other1, class _Other2,
@@ -296,13 +312,30 @@ struct pair { // store a pair of values
             noexcept(is_nothrow_constructible_v<_Ty1, const _Other1&>
                      && is_nothrow_constructible_v<_Ty2, const _Other2&>) // strengthened
         : first(_Right.first), second(_Right.second) {}
+#ifdef __cpp_lib_reference_from_temporary
+    template <class _Other1, class _Other2,
+        enable_if_t<conjunction_v<is_constructible<_Ty1, const _Other1&>, is_constructible<_Ty2, const _Other2&>>,
+            int> = 0>
+        requires reference_constructs_from_temporary_v<_Ty1, const _Other1&>
+                  || reference_constructs_from_temporary_v<_Ty2, const _Other2&>
+    explicit(!conjunction_v<is_convertible<const _Other1&, _Ty1>, is_convertible<const _Other2&, _Ty2>>)
+        pair(const pair<_Other1, _Other2>&) = delete;
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
 
     template <class _Other1, class _Other2,
         enable_if_t<conjunction_v<is_constructible<_Ty1, _Other1>, is_constructible<_Ty2, _Other2>>, int> = 0>
     constexpr explicit(!conjunction_v<is_convertible<_Other1, _Ty1>, is_convertible<_Other2, _Ty2>>)
         pair(pair<_Other1, _Other2>&& _Right) noexcept(
             is_nothrow_constructible_v<_Ty1, _Other1> && is_nothrow_constructible_v<_Ty2, _Other2>) // strengthened
         : first(_STD forward<_Other1>(_Right.first)), second(_STD forward<_Other2>(_Right.second)) {}
+#ifdef __cpp_lib_reference_from_temporary
+    template <class _Other1, class _Other2,
+        enable_if_t<conjunction_v<is_constructible<_Ty1, _Other1>, is_constructible<_Ty2, _Other2>>, int> = 0>
+        requires reference_constructs_from_temporary_v<_Ty1, _Other1&&>
+                  || reference_constructs_from_temporary_v<_Ty2, _Other2&&>
+    explicit(!conjunction_v<is_convertible<_Other1, _Ty1>, is_convertible<_Other2, _Ty2>>)
+        pair(pair<_Other1, _Other2>&&) = delete;
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
 
 #if _HAS_CXX23
     template <class _Other1, class _Other2>
@@ -312,6 +345,14 @@ struct pair { // store a pair of values
             noexcept(is_nothrow_constructible_v<_Ty1, const _Other1>
                      && is_nothrow_constructible_v<_Ty2, const _Other2>) // strengthened
         : first(_STD forward<const _Other1>(_Right.first)), second(_STD forward<const _Other2>(_Right.second)) {}
+#ifdef __cpp_lib_reference_from_temporary
+    template <class _Other1, class _Other2>
+        requires is_constructible_v<_Ty1, const _Other1> && is_constructible_v<_Ty2, const _Other2>
+                  && (reference_constructs_from_temporary_v<_Ty1, const _Other1 &&>
+                      || reference_constructs_from_temporary_v<_Ty2, const _Other2 &&>)
+    explicit(!conjunction_v<is_convertible<const _Other1, _Ty1>, is_convertible<const _Other2, _Ty2>>)
+        pair(const pair<_Other1, _Other2>&&) = delete;
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
 
 #ifdef __EDG__ // TRANSITION, VSO-1900279
     template <class _Other, enable_if_t<_Can_construct_from_pair_like<_Other, _Ty1, _Ty2>, int> = 0>
@@ -326,6 +367,15 @@ struct pair { // store a pair of values
                  && is_nothrow_constructible_v<_Ty2, decltype(_STD get<1>(_STD declval<_Other>()))>) // strengthened
         : first(_STD get<0>(_STD forward<_Other>(_Right))), second(_STD get<1>(_STD forward<_Other>(_Right))) {
     }
+#ifdef __cpp_lib_reference_from_temporary
+    template <_Pair_like_non_subrange _Other>
+        requires conjunction_v<is_constructible<_Ty1, decltype(_STD get<0>(_STD declval<_Other>()))>,
+                     is_constructible<_Ty2, decltype(_STD get<1>(_STD declval<_Other>()))>>
+                  && (reference_constructs_from_temporary_v<_Ty1, decltype(_STD get<0>(_STD declval<_Other>()))>
+                      || reference_constructs_from_temporary_v<_Ty2, decltype(_STD get<1>(_STD declval<_Other>()))>)
+    explicit(!conjunction_v<is_convertible<decltype(_STD get<0>(_STD declval<_Other>())), _Ty1>,
+        is_convertible<decltype(_STD get<1>(_STD declval<_Other>())), _Ty2>>) pair(_Other&&) = delete;
+#endif // ^^^ defined(__cpp_lib_reference_from_temporary) ^^^
 #endif // _HAS_CXX23
 
     template <class... _Types1, class... _Types2>

stl/inc/yvals_core.h

@@ -364,6 +364,8 @@
 // P2165R4 Compatibility Between tuple, pair, And tuple-like Objects
 // P2166R1 Prohibiting basic_string And basic_string_view Construction From nullptr
 // P2186R2 Removing Garbage Collection Support
+// P2255R2 Type Traits To Detect References Binding To Temporaries
+//     (for Clang only)
 // P2273R3 constexpr unique_ptr
 // P2278R4 cbegin Should Always Return A Constant Iterator
 // P2286R8 Formatting Ranges
@@ -1829,28 +1831,34 @@ _EMIT_STL_ERROR(STL1004, "C++98 unexpected() is incompatible with C++23 unexpect
 #define __cpp_lib_is_implicit_lifetime 202302L
 #endif // ^^^ no workaround ^^^
 
-#define __cpp_lib_is_scoped_enum              202011L
-#define __cpp_lib_mdspan                      202207L
-#define __cpp_lib_move_only_function          202110L
-#define __cpp_lib_out_ptr                     202311L
-#define __cpp_lib_print                       202406L
-#define __cpp_lib_ranges_as_const             202311L
-#define __cpp_lib_ranges_as_rvalue            202207L
-#define __cpp_lib_ranges_cartesian_product    202207L
-#define __cpp_lib_ranges_chunk                202202L
-#define __cpp_lib_ranges_chunk_by             202202L
-#define __cpp_lib_ranges_contains             202207L
-#define __cpp_lib_ranges_enumerate            202302L
-#define __cpp_lib_ranges_find_last            202207L
-#define __cpp_lib_ranges_fold                 202207L
-#define __cpp_lib_ranges_iota                 202202L
-#define __cpp_lib_ranges_join_with            202202L
-#define __cpp_lib_ranges_repeat               202207L
-#define __cpp_lib_ranges_slide                202202L
-#define __cpp_lib_ranges_starts_ends_with     202106L
-#define __cpp_lib_ranges_stride               202207L
-#define __cpp_lib_ranges_to_container         202202L
-#define __cpp_lib_ranges_zip                  202110L
+#define __cpp_lib_is_scoped_enum           202011L
+#define __cpp_lib_mdspan                   202207L
+#define __cpp_lib_move_only_function       202110L
+#define __cpp_lib_out_ptr                  202311L
+#define __cpp_lib_print                    202406L
+#define __cpp_lib_ranges_as_const          202311L
+#define __cpp_lib_ranges_as_rvalue         202207L
+#define __cpp_lib_ranges_cartesian_product 202207L
+#define __cpp_lib_ranges_chunk             202202L
+#define __cpp_lib_ranges_chunk_by          202202L
+#define __cpp_lib_ranges_contains          202207L
+#define __cpp_lib_ranges_enumerate         202302L
+#define __cpp_lib_ranges_find_last         202207L
+#define __cpp_lib_ranges_fold              202207L
+#define __cpp_lib_ranges_iota              202202L
+#define __cpp_lib_ranges_join_with         202202L
+#define __cpp_lib_ranges_repeat            202207L
+#define __cpp_lib_ranges_slide             202202L
+#define __cpp_lib_ranges_starts_ends_with  202106L
+#define __cpp_lib_ranges_stride            202207L
+#define __cpp_lib_ranges_to_container      202202L
+#define __cpp_lib_ranges_zip               202110L
+
+// TRANSITION, MSVC and EDG haven't implemented intrinsics needed for P2255R2.
+#if defined(__clang__) && !defined(__EDG__)
+#define __cpp_lib_reference_from_temporary 202202L
+#endif // ^^^ no workaround ^^^
+
 #define __cpp_lib_spanstream                  202106L
 #define __cpp_lib_stacktrace                  202011L
 #define __cpp_lib_stdatomic_h                 202011L

tests/libcxx/expected_results.txt

@@ -156,6 +156,10 @@ std/utilities/utility/mem.res/mem.res.pool/mem.res.pool.mem/unsync_allocate.pass
 std/utilities/utility/mem.res/mem.res.pool/mem.res.pool.mem/unsync_allocate_overaligned_request.pass.cpp FAIL
 std/utilities/utility/mem.res/mem.res.pool/mem.res.pool.mem/unsync_deallocate_matches_allocate.pass.cpp FAIL
 
+# libc++ hasn't completely implemented P2255R2 "Type Traits To Detect References Binding To Temporaries"
+std/utilities/meta/meta.rel/is_invocable_r.compile.pass.cpp:2 FAIL
+std/utilities/meta/meta.rel/is_invocable_r_v.compile.pass.cpp:2 FAIL
+
 # libc++ is missing various Ranges DRs
 std/language.support/support.limits/support.limits.general/memory.version.compile.pass.cpp FAIL
 std/language.support/support.limits/support.limits.general/ranges.version.compile.pass.cpp FAIL
@@ -562,9 +566,12 @@ std/containers/container.adaptors/flat.set/types.compile.pass.cpp FAIL
 std/language.support/support.limits/support.limits.general/flat_set.version.compile.pass.cpp FAIL
 
 # P2255R2 "Type Traits To Detect References Binding To Temporaries"
+# (type_traits.version.compile.pass.cpp still fails for Clang due to LLVM-48860)
 std/language.support/support.limits/support.limits.general/type_traits.version.compile.pass.cpp FAIL
-std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp FAIL
-std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp FAIL
+std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp:0 FAIL
+std/utilities/meta/meta.unary/meta.unary.prop/reference_constructs_from_temporary.pass.cpp:1 FAIL
+std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp:0 FAIL
+std/utilities/meta/meta.unary/meta.unary.prop/reference_converts_from_temporary.pass.cpp:1 FAIL
 std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp:0 FAIL
 std/utilities/tuple/tuple.tuple/tuple.cnstr/PR20855_tuple_ref_binding_diagnostics.pass.cpp:1 FAIL
 

tests/std/test.lst

@@ -636,6 +636,10 @@ tests\P2165R4_tuple_like_pair
 tests\P2165R4_tuple_like_relational_operators
 tests\P2165R4_tuple_like_tuple_members
 tests\P2231R1_complete_constexpr_optional_variant
+tests\P2255R2_invocation
+tests\P2255R2_reference_constructs_from_temporary
+tests\P2255R2_reference_converts_from_temporary
+tests\P2255R2_tuple_pair_construction
 tests\P2273R3_constexpr_unique_ptr
 tests\P2278R4_basic_const_iterator
 tests\P2278R4_const_span

tests/std/tests/P2255R2_invocation/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst

tests/std/tests/P2255R2_invocation/test.compile.pass.cpp

@@ -0,0 +1,64 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <functional>
+#include <type_traits>
+
+using namespace std;
+
+// TRANSITION, MSVC and EDG haven't implemented intrinsics needed for P2255R2.
+#ifdef __cpp_lib_reference_from_temporary
+struct LvalueTempFunctor {
+    int operator()() const& noexcept;
+    const int& operator()() const&& noexcept;
+};
+
+static_assert(is_invocable_r_v<const int&, int& (*) ()>);
+static_assert(!is_invocable_r_v<const int&, short& (*) ()>);
+static_assert(!is_invocable_r_v<const int&, int (*)()>);
+static_assert(!is_invocable_r_v<int&&, int (*)()>);
+static_assert(is_invocable_r_v<int, int (*)()>);
+static_assert(is_invocable_r_v<const int&, LvalueTempFunctor>);
+static_assert(!is_invocable_r_v<const int&, LvalueTempFunctor&>);
+
+static_assert(is_constructible_v<function<const int&()>, int& (*) ()>);
+static_assert(!is_constructible_v<function<const int&()>, short& (*) ()>);
+static_assert(!is_constructible_v<function<const int&()>, int (*)()>);
+static_assert(!is_constructible_v<function<const int&()>, LvalueTempFunctor>);
+
+static_assert(is_constructible_v<move_only_function<const int&()>, int& (*) ()>);
+static_assert(!is_constructible_v<move_only_function<const int&()>, short& (*) ()>);
+static_assert(!is_constructible_v<move_only_function<const int&()>, int (*)()>);
+
+static_assert(is_constructible_v<move_only_function<const int&() const>, int& (*) ()>);
+static_assert(!is_constructible_v<move_only_function<const int&() const>, short& (*) ()>);
+static_assert(!is_constructible_v<move_only_function<const int&() const>, int (*)()>);
+
+static_assert(!is_constructible_v<move_only_function<const int&() &>, LvalueTempFunctor>);
+static_assert(!is_constructible_v<move_only_function<const int&() const&>, LvalueTempFunctor>);
+static_assert(is_constructible_v<move_only_function<const int&() &&>, LvalueTempFunctor>);
+static_assert(is_constructible_v<move_only_function<const int&() const&&>, LvalueTempFunctor>);
+
+#ifdef __cpp_noexcept_function_type
+static_assert(is_nothrow_invocable_r_v<const int&, int& (*) () noexcept>);
+static_assert(!is_nothrow_invocable_r_v<const int&, short& (*) () noexcept>);
+static_assert(!is_nothrow_invocable_r_v<const int&, int (*)() noexcept>);
+static_assert(!is_nothrow_invocable_r_v<int&&, int (*)() noexcept>);
+static_assert(is_nothrow_invocable_r_v<int, int (*)() noexcept>);
+static_assert(is_nothrow_invocable_r_v<const int&, LvalueTempFunctor>);
+static_assert(!is_nothrow_invocable_r_v<const int&, LvalueTempFunctor&>);
+
+static_assert(is_constructible_v<move_only_function<const int&() noexcept>, int& (*) () noexcept>);
+static_assert(!is_constructible_v<move_only_function<const int&() noexcept>, short& (*) () noexcept>);
+static_assert(!is_constructible_v<move_only_function<const int&() noexcept>, int (*)() noexcept>);
+
+static_assert(is_constructible_v<move_only_function<const int&() const noexcept>, int& (*) () noexcept>);
+static_assert(!is_constructible_v<move_only_function<const int&() const noexcept>, short& (*) () noexcept>);
+static_assert(!is_constructible_v<move_only_function<const int&() const noexcept>, int (*)() noexcept>);
+
+static_assert(!is_constructible_v<move_only_function<const int&() & noexcept>, LvalueTempFunctor>);
+static_assert(!is_constructible_v<move_only_function<const int&() const & noexcept>, LvalueTempFunctor>);
+static_assert(is_constructible_v<move_only_function<const int&() && noexcept>, LvalueTempFunctor>);
+static_assert(is_constructible_v<move_only_function<const int&() const && noexcept>, LvalueTempFunctor>);
+#endif // defined(__cpp_noexcept_function_type)
+#endif // ^^^ no workaround ^^^

tests/std/tests/P2255R2_reference_constructs_from_temporary/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst