Project: Don't require computed default dialect if compiler was forced.

Commit 7235334a (Project: Determine default language dialect for the
compiler., 2015-09-15) introduced a mechanism to determine the default
dialect used for the running compiler.  If conditions in
the <CompilerId>-<Lang>.cmake file are such that compile features for
that version of the compiler should be supported, the _DEFAULT_STANDARD
is set to the computed value.

However, the CMakeForceCompiler module allows users to bypass execution of the
compiler by CMake.  In that case, do not set the _DEFAULT_STANDARD variable at
all, which effectively disables the compile-features where the module is used.

No compile features have ever been recorded where the module is used so no
functionality is lost.

Modules/Compiler/AppleClang-C.cmake

@@ -13,10 +13,12 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
 endif()
 
 if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.0)
-  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}")
+  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})
   endif()
-  set(CMAKE_C_STANDARD_DEFAULT ${CMAKE_C_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_c_compile_features)

Modules/Compiler/AppleClang-CXX.cmake

@@ -23,10 +23,12 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.1)
 endif()
 
 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.0)
-  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}")
+  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})
   endif()
-  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 

Modules/Compiler/Clang-C.cmake

@@ -18,10 +18,12 @@ if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
 endif()
 
 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}")
+  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})
   endif()
-  set(CMAKE_C_STANDARD_DEFAULT ${CMAKE_C_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_c_compile_features)

Modules/Compiler/Clang-CXX.cmake

@@ -32,10 +32,12 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
 endif()
 
 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}")
+  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})
   endif()
-  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_cxx_compile_features)

Modules/Compiler/GNU-C.cmake

@@ -23,10 +23,12 @@ elseif (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 4.6)
 endif()
 
 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}")
+  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})
   endif()
-  set(CMAKE_C_STANDARD_DEFAULT ${CMAKE_C_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 

Modules/Compiler/GNU-CXX.cmake

@@ -35,10 +35,12 @@ elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
 endif()
 
 if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.4)
-  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}")
+  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})
   endif()
-  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_cxx_compile_features)

Modules/Compiler/SunPro-CXX.cmake

@@ -37,10 +37,12 @@ if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
 endif()
 
 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}")
+  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})
   endif()
-  set(CMAKE_CXX_STANDARD_DEFAULT ${CMAKE_CXX_STANDARD_COMPUTED_DEFAULT})
 endif()
 
 macro(cmake_record_cxx_compile_features)