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.
If no compiler feature information is known for a given compiler
version, do not set a language standard default either. The two
settings must be recorded consistently.
GNU-CXX already has complex logic and sets the _result to 0 before
tests which may set it to something else.
Change the other modules to be consistent with that.
Features are currently recorded accidentally for all versions of
AppleClang > 3.4 (I have no idea how that relates to upstream
Clang). Presumably that version has the features which are
accidentally recorded, but in the future features will be
recorded initially for only AppleClang >= 5.1, which would
appear as a feature regression.
Commit v3.1.0-rc1~635^2~11 (Don't load Clang-CXX from
AppleClang-CXX., 2013-11-11) ajusted the logic for the CXX
language. Make a similar change for the C language.
Apple distributes their own Clang build with their own version numbers
that differ from upstream Clang. Use the __apple_build_version__ symbol
to identify the Apple Clang compiler and report the Apple Build Version
as the fourth version component in CMAKE_<LANG>_COMPILER_VERSION. Add
Compiler/AppleClang-<lang> and Platform/Darwin-AppleClang-<lang> modules
that simply include the upstream equivalents.
Fix comparisons of CMAKE_<LANG>_COMPILER_ID to Clang in CMake's own
source and tests to account for AppleClang.
Stephen Kelly
Brad King