Project: Guess default standard dialect if compiler was forced (#15852)

Prior to commit v3.4.0-rc1~71^2 (Project: Determine default language
dialect for the compiler, 2015-09-15) we always guessed the default
language standard dialect based on the compiler version.  This was not
reliable so that commit switched to computing the default language
standard dialect while detecting the compiler id.

When a toolchain file uses CMakeForceCompiler to set the compiler id
then the detection does not occur.  Therefore commit v3.4.0-rc1~54^2
(Project: Don't require computed default dialect if compiler was forced,
2015-09-22) made the lack of detection an error only if the compiler was
not forced.  However, this means that projects using CMakeForceCompiler
no longer even get the guess that we had before so <LANG>_COMPILER does
not work.

Due to the sophistication of CMake's compiler detection logic projects
should be ported away from using CMakeForceCompiler.  In the meantime,
restore a guess of the default language standard dialect when the
compiler is forced.

Modules/Compiler/AppleClang-C.cmake

@@ -18,6 +18,9 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
       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.
+    set(CMAKE_C_STANDARD_DEFAULT 99)
   endif()
 endif()
 

Modules/Compiler/AppleClang-CXX.cmake

@@ -28,6 +28,9 @@ if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
       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()
 

Modules/Compiler/Clang-C.cmake

@@ -23,6 +23,13 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
       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()
 

Modules/Compiler/Clang-CXX.cmake

@@ -37,6 +37,9 @@ if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
       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()
 

Modules/Compiler/GNU-C.cmake

@@ -28,6 +28,13 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.4)
       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 5.0)
+      set(CMAKE_C_STANDARD_DEFAULT 11)
+    else()
+      set(CMAKE_C_STANDARD_DEFAULT 90)
+    endif()
   endif()
 endif()
 

Modules/Compiler/GNU-CXX.cmake

@@ -40,6 +40,9 @@ if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.4)
       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()
 

Modules/Compiler/SunPro-CXX.cmake

@@ -42,6 +42,9 @@ if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
       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()