Project: Determine default language dialect for the compiler.

Use the __cplusplus and __STDC_VERSION__ macros to automatically
determine the default dialect for the compiler while determining its
id and version.

Help/manual/cmake-developer.7.rst

@@ -52,9 +52,9 @@ When adding the first supported feature to a particular CompilerId, it is
 necessary to list support for all features known to cmake (See
 :variable:`CMAKE_C_COMPILE_FEATURES` and
 :variable:`CMAKE_CXX_COMPILE_FEATURES` as appropriate), where available for
-the compiler.  Furthermore, set ``CMAKE_<LANG>_STANDARD_DEFAULT`` to the
-default language standard level the compiler uses, or to the empty string
-if the compiler has no notion of standard levels (such as ``MSVC``).
+the compiler.  Ensure that the ``CMAKE_<LANG>_STANDARD_DEFAULT`` is set to
+the computed internal variable ``CMAKE_<LANG>_STANDARD_COMPUTED_DEFAULT``
+for compiler versions which should be supported.
 
 It is sensible to record the features for the most recent version of a
 particular CompilerId first, and then work backwards.  It is sensible to

Modules/CMakeCCompiler.cmake.in

@@ -2,6 +2,7 @@ set(CMAKE_C_COMPILER "@CMAKE_C_COMPILER@")
 set(CMAKE_C_COMPILER_ARG1 "@CMAKE_C_COMPILER_ARG1@")
 set(CMAKE_C_COMPILER_ID "@CMAKE_C_COMPILER_ID@")
 set(CMAKE_C_COMPILER_VERSION "@CMAKE_C_COMPILER_VERSION@")
+set(CMAKE_C_STANDARD_COMPUTED_DEFAULT "@CMAKE_C_STANDARD_COMPUTED_DEFAULT@")
 set(CMAKE_C_COMPILE_FEATURES "@CMAKE_C_COMPILE_FEATURES@")
 set(CMAKE_C90_COMPILE_FEATURES "@CMAKE_C90_COMPILE_FEATURES@")
 set(CMAKE_C99_COMPILE_FEATURES "@CMAKE_C99_COMPILE_FEATURES@")

Modules/CMakeCCompilerId.c.in

@@ -24,6 +24,17 @@ char const* qnxnto = "INFO" ":" "qnxnto[]";
 @CMAKE_C_COMPILER_ID_PLATFORM_CONTENT@
 @CMAKE_C_COMPILER_ID_ERROR_FOR_TEST@
 
+const char* info_language_dialect_default = "INFO" ":" "dialect_default["
+#if !defined(__STDC_VERSION__)
+  "90"
+#elif __STDC_VERSION__ >= 201000L
+  "11"
+#elif __STDC_VERSION__ >= 199901L
+  "99"
+#else
+#endif
+"]";
+
 /*--------------------------------------------------------------------------*/
 
 #ifdef ID_VOID_MAIN

Modules/CMakeCXXCompiler.cmake.in

@@ -2,6 +2,7 @@ set(CMAKE_CXX_COMPILER "@CMAKE_CXX_COMPILER@")
 set(CMAKE_CXX_COMPILER_ARG1 "@CMAKE_CXX_COMPILER_ARG1@")
 set(CMAKE_CXX_COMPILER_ID "@CMAKE_CXX_COMPILER_ID@")
 set(CMAKE_CXX_COMPILER_VERSION "@CMAKE_CXX_COMPILER_VERSION@")
+set(CMAKE_CXX_STANDARD_COMPUTED_DEFAULT "@CMAKE_CXX_STANDARD_COMPUTED_DEFAULT@")
 set(CMAKE_CXX_COMPILE_FEATURES "@CMAKE_CXX_COMPILE_FEATURES@")
 set(CMAKE_CXX98_COMPILE_FEATURES "@CMAKE_CXX98_COMPILE_FEATURES@")
 set(CMAKE_CXX11_COMPILE_FEATURES "@CMAKE_CXX11_COMPILE_FEATURES@")

Modules/CMakeCXXCompilerId.cpp.in

@@ -23,6 +23,16 @@ char const* qnxnto = "INFO" ":" "qnxnto[]";
 @CMAKE_CXX_COMPILER_ID_PLATFORM_CONTENT@
 @CMAKE_CXX_COMPILER_ID_ERROR_FOR_TEST@
 
+const char* info_language_dialect_default = "INFO" ":" "dialect_default["
+#if __cplusplus >= 201402L
+  "14"
+#elif __cplusplus >= 201103L
+  "11"
+#else
+  "98"
+#endif
+"]";
+
 /*--------------------------------------------------------------------------*/
 
 int main(int argc, char* argv[])

Modules/CMakeDetermineCompilerId.cmake

@@ -102,6 +102,7 @@ function(CMAKE_DETERMINE_COMPILER_ID lang flagvar src)
   set(CMAKE_${lang}_COMPILER_VERSION "${CMAKE_${lang}_COMPILER_VERSION}" PARENT_SCOPE)
   set(CMAKE_${lang}_SIMULATE_ID "${CMAKE_${lang}_SIMULATE_ID}" PARENT_SCOPE)
   set(CMAKE_${lang}_SIMULATE_VERSION "${CMAKE_${lang}_SIMULATE_VERSION}" PARENT_SCOPE)
+  set(CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT "${CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT}" PARENT_SCOPE)
 endfunction()
 
 include(CMakeCompilerIdDetection)
@@ -487,6 +488,9 @@ function(CMAKE_DETERMINE_COMPILER_ID_CHECK lang file)
       if("${info}" MATCHES "INFO:qnxnto\\[\\]")
         set(COMPILER_QNXNTO 1)
       endif()
+      if("${info}" MATCHES "INFO:dialect_default\\[([^]\"]*)\\]")
+        set(CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT "${CMAKE_MATCH_1}")
+      endif()
     endforeach()
 
     # Construct compiler version from components if needed.
@@ -590,6 +594,7 @@ function(CMAKE_DETERMINE_COMPILER_ID_CHECK lang file)
   set(CMAKE_${lang}_SIMULATE_VERSION "${CMAKE_${lang}_SIMULATE_VERSION}" PARENT_SCOPE)
   set(CMAKE_EXECUTABLE_FORMAT "${CMAKE_EXECUTABLE_FORMAT}" PARENT_SCOPE)
   set(COMPILER_QNXNTO "${COMPILER_QNXNTO}" PARENT_SCOPE)
+  set(CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT "${CMAKE_${lang}_STANDARD_COMPUTED_DEFAULT}" PARENT_SCOPE)
 endfunction()
 
 #-----------------------------------------------------------------------------

Modules/Compiler/AppleClang-C.cmake

@@ -13,7 +13,10 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
 endif()
 
 if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
-  set(CMAKE_C_STANDARD_DEFAULT 99)
+  if (NOT CMAKE_C_STANDARD_COMPUTED_DEFAULT)
+    message(FATAL_ERROR "CMAKE_C_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_C_COMPILER_ID} (${CMAKE_C_COMPILER}) version ${CMAKE_C_COMPILER_VERSION}")
+  endif()
+  set(CMAKE_C_STANDARD_DEFAULT ${CMAKE_C_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_c_compile_features)

Modules/Compiler/AppleClang-CXX.cmake

@@ -23,9 +23,13 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.1)
 endif()
 
 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
-  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+  if (NOT CMAKE_CXX_STANDARD_COMPUTED_DEFAULT)
+    message(FATAL_ERROR "CMAKE_CXX_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_CXX_COMPILER_ID} (${CMAKE_CXX_COMPILER}) version ${CMAKE_CXX_COMPILER_VERSION}")
+  endif()
+  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
+
 macro(cmake_record_cxx_compile_features)
   macro(_get_appleclang_features std_version list)
     record_compiler_features(CXX "${std_version}" ${list})

Modules/Compiler/Clang-C.cmake

@@ -17,10 +17,11 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
 endif()
 
-if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.6)
-  set(CMAKE_C_STANDARD_DEFAULT 11)
-elseif(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
-  set(CMAKE_C_STANDARD_DEFAULT 99)
+if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
+  if (NOT CMAKE_C_STANDARD_COMPUTED_DEFAULT)
+    message(FATAL_ERROR "CMAKE_C_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_C_COMPILER_ID} (${CMAKE_C_COMPILER}) version ${CMAKE_C_COMPILER_VERSION}")
+  endif()
+  set(CMAKE_C_STANDARD_DEFAULT ${CMAKE_C_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_c_compile_features)

Modules/Compiler/Clang-CXX.cmake

@@ -31,8 +31,11 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
 endif()
 
-if(UNIX AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
-  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
+  if (NOT CMAKE_CXX_STANDARD_COMPUTED_DEFAULT)
+    message(FATAL_ERROR "CMAKE_CXX_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_CXX_COMPILER_ID} (${CMAKE_CXX_COMPILER}) version ${CMAKE_CXX_COMPILER_VERSION}")
+  endif()
+  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_cxx_compile_features)

Modules/Compiler/GNU-C.cmake

@@ -22,12 +22,14 @@ elseif (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.6)
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu1x")
 endif()
 
-if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 5.0)
-  set(CMAKE_C_STANDARD_DEFAULT 11)
-elseif(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.4)
-  set(CMAKE_C_STANDARD_DEFAULT 90)
+if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.4)
+  if (NOT CMAKE_C_STANDARD_COMPUTED_DEFAULT)
+    message(FATAL_ERROR "CMAKE_C_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_C_COMPILER_ID} (${CMAKE_C_COMPILER}) version ${CMAKE_C_COMPILER_VERSION}")
+  endif()
+  set(CMAKE_C_STANDARD_DEFAULT ${CMAKE_C_STANDARD_COMPUTED_DEFAULT})
 endif()
 
+
 macro(cmake_record_c_compile_features)
   macro(_get_gcc_features std_version list)
     record_compiler_features(C "${std_version}" ${list})

Modules/Compiler/GNU-CXX.cmake

@@ -35,7 +35,10 @@ elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
 endif()
 
 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.4)
-  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+  if (NOT CMAKE_CXX_STANDARD_COMPUTED_DEFAULT)
+    message(FATAL_ERROR "CMAKE_CXX_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_CXX_COMPILER_ID} (${CMAKE_CXX_COMPILER}) version ${CMAKE_CXX_COMPILER_VERSION}")
+  endif()
+  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_cxx_compile_features)

Modules/Compiler/SunPro-CXX.cmake

@@ -36,8 +36,11 @@ if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
   set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=c++11")
 endif()
 
-if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
-  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
+  if (NOT CMAKE_CXX_STANDARD_COMPUTED_DEFAULT)
+    message(FATAL_ERROR "CMAKE_CXX_STANDARD_COMPUTED_DEFAULT should be set for ${CMAKE_CXX_COMPILER_ID} (${CMAKE_CXX_COMPILER}) version ${CMAKE_CXX_COMPILER_VERSION}")
+  endif()
+  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_cxx_compile_features)

Tests/RunCMake/CompileFeatures/RunCMakeTest.cmake

@@ -29,7 +29,7 @@ if (NOT CXX_FEATURES)
   run_cmake(NoSupportedCxxFeatures)
   run_cmake(NoSupportedCxxFeaturesGenex)
 else()
-  if(CXX_STANDARD_DEFAULT)
+  if(CXX_STANDARD_DEFAULT EQUAL 98)
     run_cmake(LinkImplementationFeatureCycle)
   endif()
   run_cmake(LinkImplementationFeatureCycleSolved)