Features: Add infrastructure for C++ 20 language standard

Issue: #17849

Help/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.rst

@@ -26,6 +26,9 @@ The features known to this version of CMake are:
 ``cxx_std_17``
   Compiler mode is aware of C++ 17.
 
+``cxx_std_20``
+  Compiler mode is aware of C++ 20.
+
 ``cxx_aggregate_default_initializers``
   Aggregate default initializers, as defined in N3605_.
 

Help/prop_tgt/CXX_STANDARD.rst

@@ -9,7 +9,7 @@ flag such as ``-std=gnu++11`` to the compile line.  For compilers that
 have no notion of a standard level, such as Microsoft Visual C++ before
 2015 Update 3, this has no effect.
 
-Supported values are ``98``, ``11``, ``14``, and ``17``.
+Supported values are ``98``, ``11``, ``14``, ``17``, and ``20``.
 
 If the value requested does not result in a compile flag being added for
 the compiler in use, a previous standard flag will be added instead.  This

Modules/CMakeCUDACompilerId.cu.in

@@ -17,7 +17,9 @@ char const* info_simulate = "INFO" ":" "simulate[" SIMULATE_ID "]";
 @CMAKE_CUDA_COMPILER_ID_ERROR_FOR_TEST@
 
 const char* info_language_dialect_default = "INFO" ":" "dialect_default["
-#if __cplusplus > 201402L
+#if __cplusplus > 201703L
+  "20"
+#elif __cplusplus >= 201703L
   "17"
 #elif __cplusplus >= 201402L
   "14"

Modules/CMakeCXXCompiler.cmake.in

@@ -10,6 +10,7 @@ set(CMAKE_CXX98_COMPILE_FEATURES "@CMAKE_CXX98_COMPILE_FEATURES@")
 set(CMAKE_CXX11_COMPILE_FEATURES "@CMAKE_CXX11_COMPILE_FEATURES@")
 set(CMAKE_CXX14_COMPILE_FEATURES "@CMAKE_CXX14_COMPILE_FEATURES@")
 set(CMAKE_CXX17_COMPILE_FEATURES "@CMAKE_CXX17_COMPILE_FEATURES@")
+set(CMAKE_CXX20_COMPILE_FEATURES "@CMAKE_CXX20_COMPILE_FEATURES@")
 
 set(CMAKE_CXX_PLATFORM_ID "@CMAKE_CXX_PLATFORM_ID@")
 set(CMAKE_CXX_SIMULATE_ID "@CMAKE_CXX_SIMULATE_ID@")

Modules/CMakeCXXCompilerId.cpp.in

@@ -34,7 +34,9 @@ char const *info_cray = "INFO" ":" "compiler_wrapper[CrayPrgEnv]";
 #endif
 
 const char* info_language_dialect_default = "INFO" ":" "dialect_default["
-#if CXX_STD > 201402L
+#if CXX_STD > 201703L
+  "20"
+#elif CXX_STD >= 201703L
   "17"
 #elif CXX_STD >= 201402L
   "14"

Modules/CMakeDetermineCompileFeatures.cmake

@@ -49,6 +49,7 @@ function(cmake_determine_compile_features lang)
     set(CMAKE_CXX11_COMPILE_FEATURES)
     set(CMAKE_CXX14_COMPILE_FEATURES)
     set(CMAKE_CXX17_COMPILE_FEATURES)
+    set(CMAKE_CXX20_COMPILE_FEATURES)
 
     include("${CMAKE_ROOT}/Modules/Internal/FeatureTesting.cmake")
 
@@ -59,6 +60,9 @@ function(cmake_determine_compile_features lang)
       return()
     endif()
 
+    if (CMAKE_CXX17_COMPILE_FEATURES AND CMAKE_CXX20_COMPILE_FEATURES)
+      list(REMOVE_ITEM CMAKE_CXX20_COMPILE_FEATURES ${CMAKE_CXX17_COMPILE_FEATURES})
+    endif()
     if (CMAKE_CXX14_COMPILE_FEATURES AND CMAKE_CXX17_COMPILE_FEATURES)
       list(REMOVE_ITEM CMAKE_CXX17_COMPILE_FEATURES ${CMAKE_CXX14_COMPILE_FEATURES})
     endif()
@@ -75,6 +79,7 @@ function(cmake_determine_compile_features lang)
         ${CMAKE_CXX11_COMPILE_FEATURES}
         ${CMAKE_CXX14_COMPILE_FEATURES}
         ${CMAKE_CXX17_COMPILE_FEATURES}
+        ${CMAKE_CXX20_COMPILE_FEATURES}
       )
     endif()
 
@@ -83,6 +88,7 @@ function(cmake_determine_compile_features lang)
     set(CMAKE_CXX11_COMPILE_FEATURES ${CMAKE_CXX11_COMPILE_FEATURES} PARENT_SCOPE)
     set(CMAKE_CXX14_COMPILE_FEATURES ${CMAKE_CXX14_COMPILE_FEATURES} PARENT_SCOPE)
     set(CMAKE_CXX17_COMPILE_FEATURES ${CMAKE_CXX17_COMPILE_FEATURES} PARENT_SCOPE)
+    set(CMAKE_CXX20_COMPILE_FEATURES ${CMAKE_CXX20_COMPILE_FEATURES} PARENT_SCOPE)
 
     message(STATUS "Detecting ${lang} compile features - done")
   endif()

Modules/Compiler/CMakeCommonCompilerMacros.cmake

@@ -78,6 +78,9 @@ endmacro()
 # Define to allow compile features to be automatically determined
 macro(cmake_record_cxx_compile_features)
   set(_result 0)
+  if(_result EQUAL 0 AND DEFINED CMAKE_CXX20_STANDARD_COMPILE_OPTION)
+    _record_compiler_features_cxx(20)
+  endif()
   if(_result EQUAL 0 AND DEFINED CMAKE_CXX17_STANDARD_COMPILE_OPTION)
     _record_compiler_features_cxx(17)
   endif()

Modules/Compiler/Clang-CXX.cmake

@@ -49,6 +49,8 @@ else()
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "")
   set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "")
   set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX20_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX20_EXTENSION_COMPILE_OPTION "")
 endif()
 
 if(NOT "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")

Modules/Compiler/MSVC-CXX.cmake

@@ -29,6 +29,12 @@ if ((CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.0.24215.1 AND
   # for meta-features for C++14 and above.  Override the default macro
   # to avoid doing unnecessary work.
   macro(cmake_record_cxx_compile_features)
+    if (DEFINED CMAKE_CXX20_STANDARD_COMPILE_OPTION)
+      list(APPEND CMAKE_CXX20_COMPILE_FEATURES cxx_std_20)
+    endif()
+    # The main cmake_record_cxx_compile_features macro makes all
+    # these conditional on CMAKE_CXX##_STANDARD_COMPILE_OPTION,
+    # but we can skip the conditions because we set them above.
     list(APPEND CMAKE_CXX17_COMPILE_FEATURES cxx_std_17)
     list(APPEND CMAKE_CXX14_COMPILE_FEATURES cxx_std_14)
     list(APPEND CMAKE_CXX98_COMPILE_FEATURES cxx_std_11) # no flag needed for 11
@@ -46,6 +52,8 @@ elseif (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "")
   set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "")
   set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX20_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX20_EXTENSION_COMPILE_OPTION "")
 
   # There is no meaningful default for this
   set(CMAKE_CXX_STANDARD_DEFAULT "")
@@ -60,6 +68,7 @@ elseif (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
       cxx_std_11
       cxx_std_14
       cxx_std_17
+      cxx_std_20
       )
     _record_compiler_features(CXX "" CMAKE_CXX_COMPILE_FEATURES)
   endmacro()

Source/cmLocalGenerator.cxx

@@ -1564,6 +1564,7 @@ void cmLocalGenerator::AddCompilerRequirementFlag(
   static std::map<std::string, std::vector<std::string>> langStdMap;
   if (langStdMap.empty()) {
     // Maintain sorted order, most recent first.
+    langStdMap["CXX"].push_back("20");
     langStdMap["CXX"].push_back("17");
     langStdMap["CXX"].push_back("14");
     langStdMap["CXX"].push_back("11");

Source/cmMakefile.cxx

@@ -4223,7 +4223,7 @@ static const char* const CXX_FEATURES[] = { nullptr FOR_EACH_CXX_FEATURE(
 #undef FEATURE_STRING
 
 static const char* const C_STANDARDS[] = { "90", "99", "11" };
-static const char* const CXX_STANDARDS[] = { "98", "11", "14", "17" };
+static const char* const CXX_STANDARDS[] = { "98", "11", "14", "17", "20" };
 
 bool cmMakefile::AddRequiredTargetFeature(cmTarget* target,
                                           const std::string& feature,
@@ -4473,8 +4473,9 @@ bool cmMakefile::HaveCxxStandardAvailable(cmTarget const* target,
   bool needCxx11 = false;
   bool needCxx14 = false;
   bool needCxx17 = false;
+  bool needCxx20 = false;
   this->CheckNeededCxxLanguage(feature, needCxx98, needCxx11, needCxx14,
-                               needCxx17);
+                               needCxx17, needCxx20);
 
   const char* existingCxxStandard = target->GetProperty("CXX_STANDARD");
   if (!existingCxxStandard) {
@@ -4494,7 +4495,8 @@ bool cmMakefile::HaveCxxStandardAvailable(cmTarget const* target,
 
   /* clang-format off */
   const char* const* needCxxLevel =
-    needCxx17 ? &CXX_STANDARDS[3]
+    needCxx20 ? &CXX_STANDARDS[4]
+    : needCxx17 ? &CXX_STANDARDS[3]
     : needCxx14 ? &CXX_STANDARDS[2]
     : needCxx11 ? &CXX_STANDARDS[1]
     : needCxx98 ? &CXX_STANDARDS[0]
@@ -4506,7 +4508,8 @@ bool cmMakefile::HaveCxxStandardAvailable(cmTarget const* target,
 
 void cmMakefile::CheckNeededCxxLanguage(const std::string& feature,
                                         bool& needCxx98, bool& needCxx11,
-                                        bool& needCxx14, bool& needCxx17) const
+                                        bool& needCxx14, bool& needCxx17,
+                                        bool& needCxx20) const
 {
   if (const char* propCxx98 =
         this->GetDefinition("CMAKE_CXX98_COMPILE_FEATURES")) {
@@ -4532,6 +4535,12 @@ void cmMakefile::CheckNeededCxxLanguage(const std::string& feature,
     cmSystemTools::ExpandListArgument(propCxx17, props);
     needCxx17 = std::find(props.begin(), props.end(), feature) != props.end();
   }
+  if (const char* propCxx20 =
+        this->GetDefinition("CMAKE_CXX20_COMPILE_FEATURES")) {
+    std::vector<std::string> props;
+    cmSystemTools::ExpandListArgument(propCxx20, props);
+    needCxx20 = std::find(props.begin(), props.end(), feature) != props.end();
+  }
 }
 
 bool cmMakefile::AddRequiredTargetCxxFeature(cmTarget* target,
@@ -4542,9 +4551,10 @@ bool cmMakefile::AddRequiredTargetCxxFeature(cmTarget* target,
   bool needCxx11 = false;
   bool needCxx14 = false;
   bool needCxx17 = false;
+  bool needCxx20 = false;
 
   this->CheckNeededCxxLanguage(feature, needCxx98, needCxx11, needCxx14,
-                               needCxx17);
+                               needCxx17, needCxx20);
 
   const char* existingCxxStandard = target->GetProperty("CXX_STANDARD");
   const char* const* existingCxxLevel = nullptr;
@@ -4589,7 +4599,8 @@ bool cmMakefile::AddRequiredTargetCxxFeature(cmTarget* target,
 
   /* clang-format off */
   const char* const* needCxxLevel =
-    needCxx17 ? &CXX_STANDARDS[3]
+    needCxx20 ? &CXX_STANDARDS[4]
+    : needCxx17 ? &CXX_STANDARDS[3]
     : needCxx14 ? &CXX_STANDARDS[2]
     : needCxx11 ? &CXX_STANDARDS[1]
     : needCxx98 ? &CXX_STANDARDS[0]

Source/cmMakefile.h

@@ -991,7 +991,7 @@ private:
                             bool& needC99, bool& needC11) const;
   void CheckNeededCxxLanguage(const std::string& feature, bool& needCxx98,
                               bool& needCxx11, bool& needCxx14,
-                              bool& needCxx17) const;
+                              bool& needCxx17, bool& needCxx20) const;
 
   bool HaveCStandardAvailable(cmTarget const* target,
                               const std::string& feature) const;

Source/cmake.h

@@ -574,6 +574,7 @@ private:
   F(cxx_std_11)                                                               \
   F(cxx_std_14)                                                               \
   F(cxx_std_17)                                                               \
+  F(cxx_std_20)                                                               \
   F(cxx_aggregate_default_initializers)                                       \
   F(cxx_alias_templates)                                                      \
   F(cxx_alignas)                                                              \

Tests/CompileFeatures/CMakeLists.txt

@@ -280,6 +280,7 @@ if (CMAKE_CXX_COMPILE_FEATURES)
     if (std_flag_idx EQUAL -1)
       add_executable(default_dialect default_dialect.cpp)
       target_compile_definitions(default_dialect PRIVATE
+        DEFAULT_CXX20=$<EQUAL:${CMAKE_CXX_STANDARD_DEFAULT},20>
         DEFAULT_CXX17=$<EQUAL:${CMAKE_CXX_STANDARD_DEFAULT},17>
         DEFAULT_CXX14=$<EQUAL:${CMAKE_CXX_STANDARD_DEFAULT},14>
         DEFAULT_CXX11=$<EQUAL:${CMAKE_CXX_STANDARD_DEFAULT},11>
@@ -434,6 +435,7 @@ else()
       HAVE_CXX_STD_11=$<COMPILE_FEATURES:cxx_std_11>
       HAVE_CXX_STD_14=$<COMPILE_FEATURES:cxx_std_14>
       HAVE_CXX_STD_17=$<COMPILE_FEATURES:cxx_std_17>
+      HAVE_CXX_STD_20=$<COMPILE_FEATURES:cxx_std_20>
     )
   endif()
 

Tests/CompileFeatures/default_dialect.cpp

@@ -8,7 +8,11 @@ struct Outputter;
 #define CXX_STD __cplusplus
 #endif
 
-#if DEFAULT_CXX17
+#if DEFAULT_CXX20
+#if CXX_STD <= 201703L
+Outputter<CXX_STD> o;
+#endif
+#elif DEFAULT_CXX17
 #if CXX_STD <= 201402L
 Outputter<CXX_STD> o;
 #endif