Clang: Use common compiler macros for language standard default

Modules/Compiler/Clang-C.cmake

@@ -17,21 +17,7 @@ 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 3.4)
-  if (NOT CMAKE_C_COMPILER_FORCED)
-    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})
-  elseif(NOT DEFINED CMAKE_C_STANDARD_DEFAULT)
-    # Compiler id was forced so just guess the default standard level.
-    if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.6)
-      set(CMAKE_C_STANDARD_DEFAULT 11)
-    else()
-      set(CMAKE_C_STANDARD_DEFAULT 99)
-    endif()
-  endif()
-endif()
+__compiler_check_default_language_standard(C 3.4 99 3.6 11)
 
 macro(cmake_record_c_compile_features)
   set(_result 0)

Modules/Compiler/Clang-CXX.cmake

@@ -36,17 +36,7 @@ if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
   set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++1z")
 endif()
 
-if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.1)
-  if (NOT CMAKE_CXX_COMPILER_FORCED)
-    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})
-  elseif(NOT DEFINED CMAKE_CXX_STANDARD_DEFAULT)
-    # Compiler id was forced so just guess the default standard level.
-    set(CMAKE_CXX_STANDARD_DEFAULT 98)
-  endif()
-endif()
+__compiler_check_default_language_standard(CXX 3.1 98)
 
 macro(cmake_record_cxx_compile_features)
   set(_result 0)

Modules/Compiler/Clang.cmake

@@ -8,6 +8,8 @@ if(__COMPILER_CLANG)
 endif()
 set(__COMPILER_CLANG 1)
 
+include(Compiler/CMakeCommonCompilerMacros)
+
 if("x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC"
     OR "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
   macro(__compiler_clang lang)