P0627R6: Function to mark unreachable code

stl/inc/utility

@@ -750,6 +750,14 @@ template <class _Ty>
 _NODISCARD constexpr underlying_type_t<_Ty> to_underlying(_Ty _Value) noexcept {
     return static_cast<underlying_type_t<_Ty>>(_Value);
 }
+
+[[noreturn]] inline void unreachable() noexcept /* strengthened */ {
+#ifdef __clang__
+    __builtin_unreachable();
+#else // ^^^ __clang__ ^^^ // vvv not __clang__ vvv
+    __assume(false);
+#endif // ^^^ not __clang__ ^^^
+}
 #endif // _HAS_CXX23
 
 #if _HAS_TR1_NAMESPACE

stl/inc/yvals_core.h

@@ -283,6 +283,7 @@
 // P0288R9 move_only_function
 // P0401R6 Providing Size Feedback In The Allocator Interface
 // P0448R4 <spanstream>
+// P0627R6 Function To Mark Unreachable Code
 // P0798R8 Monadic Operations For optional
 // P0943R6 Supporting C Atomics In C++
 // P1048R1 is_scoped_enum
@@ -1418,6 +1419,7 @@
 #define __cpp_lib_string_contains             202011L
 #define __cpp_lib_string_resize_and_overwrite 202110L
 #define __cpp_lib_to_underlying               202102L
+#define __cpp_lib_unreachable                 202202L
 #endif // _HAS_CXX23
 
 #define __cpp_lib_experimental_erase_if   201411L

tests/std/test.lst

@@ -281,6 +281,7 @@ tests\P0595R2_is_constant_evaluated
 tests\P0607R0_inline_variables
 tests\P0608R3_improved_variant_converting_constructor
 tests\P0616R0_using_move_in_numeric
+tests\P0627R6_unreachable
 tests\P0631R8_numbers_math_constants
 tests\P0645R10_text_formatting_args
 tests\P0645R10_text_formatting_custom_formatting

tests/std/tests/P0627R6_unreachable/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/P0627R6_unreachable/test.cpp

@@ -0,0 +1,29 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <utility>
+
+constexpr int test_impl(int arg) {
+    switch (arg) {
+    case 1:
+        return 'x';
+
+    case 2:
+        return 'y';
+
+    default:
+        std::unreachable();
+    }
+}
+
+constexpr bool test() {
+    assert(test_impl(1) == 'x');
+    assert(test_impl(2) == 'y');
+    return true;
+}
+
+int main() {
+    test();
+    static_assert(test());
+}

tests/std/tests/VSO_0157762_feature_test_macros/test.compile.pass.cpp

@@ -1818,6 +1818,20 @@ STATIC_ASSERT(__cpp_lib_uncaught_exceptions == 201411L);
 STATIC_ASSERT(__cpp_lib_unordered_map_try_emplace == 201411L);
 #endif
 
+#if _HAS_CXX23
+#ifndef __cpp_lib_unreachable
+#error __cpp_lib_unreachable is not defined
+#elif __cpp_lib_unreachable != 202202L
+#error __cpp_lib_unreachable is not 202202L
+#else
+STATIC_ASSERT(__cpp_lib_unreachable == 202202L);
+#endif
+#else
+#ifdef __cpp_lib_unreachable
+#error __cpp_lib_unreachable is defined
+#endif
+#endif
+
 #if _HAS_CXX20
 #ifndef __cpp_lib_unwrap_ref
 #error __cpp_lib_unwrap_ref is not defined