P1682R3 to_underlying() For Enumerations (#1828)

Co-authored-by: Casey Carter <cartec69@gmail.com>
Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/utility

@@ -763,6 +763,13 @@ _NODISCARD constexpr bool in_range(const _Ty _Value) noexcept {
 }
 #endif // _HAS_CXX20
 
+#if _HAS_CXX23
+template <class _Ty>
+_NODISCARD constexpr underlying_type_t<_Ty> to_underlying(_Ty _Value) noexcept {
+    return static_cast<underlying_type_t<_Ty>>(_Value);
+}
+#endif // _HAS_CXX23
+
 #if _HAS_TR1_NAMESPACE
 namespace _DEPRECATE_TR1_NAMESPACE tr1 {
     using _STD get;

stl/inc/yvals_core.h

@@ -259,6 +259,7 @@
 // _HAS_CXX23 directly controls:
 // P1048R1 is_scoped_enum
 // P1679R3 contains() For basic_string/basic_string_view
+// P1682R3 to_underlying() For Enumerations
 
 // Parallel Algorithms Notes
 // C++ allows an implementation to implement parallel algorithms as calls to the serial algorithms.
@@ -1344,6 +1345,7 @@
 #if _HAS_CXX23
 #define __cpp_lib_is_scoped_enum  202011L
 #define __cpp_lib_string_contains 202011L
+#define __cpp_lib_to_underlying   202102L
 #endif // _HAS_CXX23
 
 // EXPERIMENTAL

tests/std/test.lst

@@ -419,6 +419,7 @@ tests\P1423R3_char8_t_remediation
 tests\P1502R1_standard_library_header_units
 tests\P1614R2_spaceship
 tests\P1645R1_constexpr_numeric
+tests\P1682R3_to_underlying
 tests\VSO_0000000_allocator_propagation
 tests\VSO_0000000_any_calling_conventions
 tests\VSO_0000000_c_math_functions

tests/std/tests/P1682R3_to_underlying/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/P1682R3_to_underlying/test.cpp

@@ -0,0 +1,51 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <type_traits>
+#include <utility>
+
+using namespace std;
+
+// TRANSITION, GH-602
+// template <class T>
+// constexpr bool can_underlying = requires(T e) { to_underlying(e); };
+
+template <class, class = void>
+constexpr bool can_underlying = false;
+template <class T>
+constexpr bool can_underlying<T, void_t<decltype(to_underlying(declval<T>()))>> = true;
+
+enum enum1 : char { a = '1' };
+enum class enum2 : int { b = 2 };
+enum class enum3 : long { c = 3 };
+enum class enum4 { d = 4 };
+
+struct not_an_enum {};
+
+static_assert(can_underlying<enum1>);
+static_assert(can_underlying<enum2>);
+static_assert(can_underlying<enum3>);
+static_assert(can_underlying<enum4>);
+static_assert(!can_underlying<not_an_enum>);
+static_assert(!can_underlying<int>);
+
+static_assert(is_same_v<decltype(to_underlying(enum1::a)), char>);
+static_assert(is_same_v<decltype(to_underlying(enum2::b)), int>);
+static_assert(is_same_v<decltype(to_underlying(enum3::c)), long>);
+static_assert(is_same_v<decltype(to_underlying(enum4::d)), int>);
+
+static_assert(noexcept(to_underlying(enum1::a)));
+
+constexpr bool test() {
+    assert(to_underlying(enum1::a) == '1');
+    assert(to_underlying(enum2::b) == 2);
+    assert(to_underlying(enum3::c) == 3);
+    assert(to_underlying(enum4::d) == 4);
+    return true;
+}
+
+int main() {
+    test();
+    static_assert(test());
+}

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

@@ -1563,6 +1563,20 @@ STATIC_ASSERT(__cpp_lib_to_chars == 201611L);
 #endif
 #endif
 
+#if _HAS_CXX23
+#ifndef __cpp_lib_to_underlying
+#error __cpp_lib_to_underlying is not defined
+#elif __cpp_lib_to_underlying != 202102L
+#error __cpp_lib_to_underlying is not 202102L
+#else
+STATIC_ASSERT(__cpp_lib_to_underlying == 202102L);
+#endif
+#else
+#ifdef __cpp_lib_to_underlying
+#error __cpp_lib_to_underlying is defined
+#endif
+#endif
+
 #ifndef __cpp_lib_transformation_trait_aliases
 #error __cpp_lib_transformation_trait_aliases is not defined
 #elif __cpp_lib_transformation_trait_aliases != 201304L