Merge topic 'fix-C-standard-features'

fb3487a9 Features: Fix C90 feature detection.
6027798a Features: Allow setting standard dialect below the default.
9d767810 Features: Populate CMAKE_<LANG>_STANDARD_DEFAULT only for supported compilers.
72537e44 Features: Add dialect compile flags only if default is known.
82c9d686 AppleClang: Remove redundant UNIX condition.

Modules/Compiler/AppleClang-C-FeatureTests.cmake

@@ -7,5 +7,5 @@ set(AppleClang_C99 "${_cmake_oldestSupported} && defined(__STDC_VERSION__) && __
 set(_cmake_feature_test_c_restrict "${AppleClang_C99}")
 set(_cmake_feature_test_c_variadic_macros "${AppleClang_C99}")
 
-set(AppleClang_C90 "${_cmake_oldestSupported} && !defined(__STDC_VERSION__)")
+set(AppleClang_C90 "${_cmake_oldestSupported}")
 set(_cmake_feature_test_c_function_prototypes "${AppleClang_C90}")

Modules/Compiler/AppleClang-C.cmake

@@ -12,7 +12,9 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
 endif()
 
-set(CMAKE_C_STANDARD_DEFAULT 99)
+if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
+  set(CMAKE_C_STANDARD_DEFAULT 99)
+endif()
 
 macro(cmake_record_c_compile_features)
   macro(_get_appleclang_features std_version list)
@@ -20,7 +22,7 @@ macro(cmake_record_c_compile_features)
   endmacro()
 
   set(_result 0)
-  if (UNIX AND NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
+  if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
     _get_appleclang_features(${CMAKE_C11_STANDARD_COMPILE_OPTION} CMAKE_C11_COMPILE_FEATURES)
     if (_result EQUAL 0)
       _get_appleclang_features(${CMAKE_C99_STANDARD_COMPILE_OPTION} CMAKE_C99_COMPILE_FEATURES)

Modules/Compiler/AppleClang-CXX.cmake

@@ -19,7 +19,9 @@ if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.1)
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
 endif()
 
-set(CMAKE_CXX_STANDARD_DEFAULT 98)
+if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
+  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+endif()
 
 macro(cmake_record_cxx_compile_features)
   macro(_get_appleclang_features std_version list)
@@ -27,7 +29,7 @@ macro(cmake_record_cxx_compile_features)
   endmacro()
 
   set(_result 0)
-  if (UNIX AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
+  if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
     set(_result 0)
     if(CMAKE_CXX14_STANDARD_COMPILE_OPTION)
       _get_appleclang_features(${CMAKE_CXX14_STANDARD_COMPILE_OPTION} CMAKE_CXX14_COMPILE_FEATURES)

Modules/Compiler/Clang-C-FeatureTests.cmake

@@ -7,5 +7,5 @@ set(Clang_C99 "${_cmake_oldestSupported} && defined(__STDC_VERSION__) && __STDC_
 set(_cmake_feature_test_c_restrict "${Clang_C99}")
 set(_cmake_feature_test_c_variadic_macros "${Clang_C99}")
 
-set(Clang_C90 "${_cmake_oldestSupported} && !defined(__STDC_VERSION__)")
+set(Clang_C90 "${_cmake_oldestSupported}")
 set(_cmake_feature_test_c_function_prototypes "${Clang_C90}")

Modules/Compiler/Clang-CXX.cmake

@@ -31,7 +31,9 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
 endif()
 
-set(CMAKE_CXX_STANDARD_DEFAULT 98)
+if(UNIX AND NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
+  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+endif()
 
 macro(cmake_record_cxx_compile_features)
   macro(_get_clang_features std_version list)

Modules/Compiler/GNU-C-FeatureTests.cmake

@@ -13,5 +13,5 @@ set(GNU44_C99 "(__GNUC__ * 100 + __GNUC_MINOR__) >= 404 && defined(__STDC_VERSIO
 set(_cmake_feature_test_c_restrict "${GNU44_C99}")
 set(_cmake_feature_test_c_variadic_macros "${GNU44_C99}")
 
-set(GNU_C90 "${_cmake_oldestSupported} && !defined(__STDC_VERSION__)")
+set(GNU_C90 "${_cmake_oldestSupported}")
 set(_cmake_feature_test_c_function_prototypes "${GNU_C90}")

Modules/Compiler/GNU-C.cmake

@@ -24,7 +24,7 @@ endif()
 
 if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 5.0)
   set(CMAKE_C_STANDARD_DEFAULT 11)
-else()
+elseif(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.4)
   set(CMAKE_C_STANDARD_DEFAULT 90)
 endif()
 

Modules/Compiler/GNU-CXX.cmake

@@ -34,7 +34,9 @@ elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
 endif()
 
-set(CMAKE_CXX_STANDARD_DEFAULT 98)
+if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.4)
+  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+endif()
 
 macro(cmake_record_cxx_compile_features)
   macro(_get_gcc_features std_version list)

Modules/Compiler/MSVC-CXX.cmake

@@ -1,5 +1,8 @@
-# MSVC has no specific language level or flags to change it.
-set(CMAKE_CXX_STANDARD_DEFAULT "")
+
+if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 16.0)
+  # MSVC has no specific language level or flags to change it.
+  set(CMAKE_CXX_STANDARD_DEFAULT "")
+endif()
 
 macro(cmake_record_cxx_compile_features)
   record_compiler_features(CXX "" CMAKE_CXX_COMPILE_FEATURES)

Modules/Compiler/SunPro-CXX.cmake

@@ -37,7 +37,9 @@ if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
   set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=c++11")
 endif()
 
-set(CMAKE_CXX_STANDARD_DEFAULT 98)
+if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
+  set(CMAKE_CXX_STANDARD_DEFAULT 98)
+endif()
 
 macro(cmake_record_cxx_compile_features)
   macro(_get_solaris_studio_features std_version list)