Utility: add a macro for conditional compilation

This is useful for future SIMD support in Magnum vectors.

v2: Add test.

src/Corrade/Utility/Test/CMakeLists.txt

@@ -234,6 +234,16 @@ if(NOT CMAKE_CXX_FLAGS MATCHES "-std=")
         corrade_add_test(UtilityMacrosCpp20Test MacrosCpp20Test.cpp)
         set_target_properties(UtilityMacrosCpp20Test PROPERTIES CORRADE_CXX_STANDARD 20)
     endif()
+
+    corrade_add_test(UtilityTypeTraitsCpp20Test TypeTraitsCpp20Test.cpp)
+    set_target_properties(UtilityMacrosCpp20Test PROPERTIES CORRADE_CXX_STANDARD 20)
+
+    if((CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "9.0") OR
+       (CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "9.0") OR
+       (CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "11.4") OR
+       (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC" AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS "19.25"))
+        set_target_properties(UtilityTypeTraitsCpp20Test PROPERTIES CORRADE_CXX_STANDARD 14)
+    endif()
 endif()
 
 # It should return with a non-zero exit code

src/Corrade/Utility/Test/TypeTraitsCpp20Test.cpp

@@ -0,0 +1,68 @@
+/*
+    This file is part of Corrade.
+
+    Copyright © 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016,
+                2017, 2018, 2019, 2020, 2021, 2022
+              Vladimír Vondruš <[email protected]>
+
+    Permission is hereby granted, free of charge, to any person obtaining a
+    copy of this software and associated documentation files (the "Software"),
+    to deal in the Software without restriction, including without limitation
+    the rights to use, copy, modify, merge, publish, distribute, sublicense,
+    and/or sell copies of the Software, and to permit persons to whom the
+    Software is furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included
+    in all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+    THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+    FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+    DEALINGS IN THE SOFTWARE.
+*/
+
+#include "Corrade/TestSuite/Tester.h"
+#include "Corrade/Utility/TypeTraits.h"
+#include <cstdio>
+
+namespace Corrade { namespace Utility { namespace Test { namespace {
+
+struct TypeTraitsCpp20Test: TestSuite::Tester {
+    explicit TypeTraitsCpp20Test();
+    void isConstantEvaluatedTest();
+};
+
+TypeTraitsCpp20Test::TypeTraitsCpp20Test() {
+    addTests({&TypeTraitsCpp20Test::isConstantEvaluatedTest});
+}
+
+#ifdef CORRADE_IS_CONSTANT_EVALUATED
+constexpr int constevalHelper(int i) {
+    if (CORRADE_IS_CONSTANT_EVALUATED)
+        return i + 1;
+    else {
+        std::printf("");
+        return i + 2;
+    }
+}
+#endif
+
+void TypeTraitsCpp20Test::isConstantEvaluatedTest() {
+#ifndef CORRADE_IS_CONSTANT_EVALUATED
+    CORRADE_SKIP("CORRADE_IS_CONSTANT_EVALUATED not supported on this compiler.");
+#else
+    constexpr int retConstant = constevalHelper(0);
+    static_assert(retConstant == 1, "");
+    CORRADE_COMPARE(retConstant, 1);
+    const volatile int arg = 0;
+    const int retRuntime = constevalHelper(arg);
+    CORRADE_COMPARE(retRuntime, 2);
+#endif
+}
+
+}}}}
+
+CORRADE_TEST_MAIN(Corrade::Utility::Test::TypeTraitsCpp20Test)

src/Corrade/Utility/TypeTraits.h

@@ -168,6 +168,26 @@ template<class U> class className {                                         \
         enum: bool { value = sizeof(get(std::declval<U>())) == sizeof(char) }; \
 }
 
+/** @hideinitializer
+@brief C++20 is_constant_evaluated
+@m_since{2022,10}
+
+Expands to a predicate determining whether given @cpp constexpr @ce function
+is being evaluated at compile-time or not. Under C++14 rules, constexpr
+functions may be defined as long as at least some of the code paths are able
+to be executed at compile-time. As long as a fallback is present, features
+such as SIMD or inline assembly may be used in functions marked constexpr.
+
+This support is available on all C++20 capable compilers, but also certain
+ones (Clang 9, GCC 9, MSVC 2022 17.1) that expose the feature as a non-portable
+extension. In which case it may be used under C++14 relaxed constexpr rules.
+*/
+#if (defined(CORRADE_TARGET_CLANG) && !defined(CORRADE_TARGET_APPLE_CLANG) && __clang_major__ >= 9) || (defined(CORRADE_TARGET_APPLE_CLANG) && __clang_major__*100 + __clang_minor__ >= 1104) || (defined(CORRADE_TARGET_GCC) && __GNUC__ >= 9) || (defined(CORRADE_TARGET_MSVC) && _MSC_VER >= 1931)
+#define CORRADE_IS_CONSTANT_EVALUATED (__builtin_is_constant_evaluated())
+#elif CORRADE_CXX_STANDARD >= 202002
+#define CORRADE_IS_CONSTANT_EVALUATED (std::is_constant_evaluated())
+#endif
+
 namespace Implementation {
     /* As of Eigen 3.4.0, due to these two commits in particular,
         https://gitlab.com/libeigen/eigen/-/commit/c0ca8a9fa3e03ad7ecb270adfe760a1bff7c0829