Provide two new CMake cache variables -- CLANG_DEFAULT_STD_C
and CLANG_DEFAULT_STD_CXX -- that can be used to override the default
C/ObjC and C++/ObjC++ standards appropriately. They can be set to one of
the identifiers from LangStandards.def, or left unset (the default) to
respect the current platform default.
This option is mostly intended for compiler vendors that do not wish to
continue defaulting to C++98. We are preparing a new Gentoo release that
requires GCC 6, and therefore defaults to -std=gnu++14. Since we want to
maintain as much compatibility between GCC and clang by default, we
would like to adjust the default for clang appropriately.
The code relies on explicit identifiers rather than the string aliases
for simplicity. This saves us from the necessity of parsing aliases at
build-time or adding additional processing at runtime. For the latter
case, it also adds trivial value check -- if incorrect value is passed,
the code simply fails to compile through referencing an undefined
If the variable is used to redefine the default standard, the explicit
value overrides the special case for PS4. It is done this way mostly
following other kinds of variables where 'platform defaults' are
I'm going to submit more test fixes that add explicit -std=
whenever necessary once this is approved.