Since commit v3.9.0-rc1~17^2~4 (Compilers: Port to use default
cmake_record_lang_compile_features macros, 2017-05-10) we run full
per-language-standard feature detection for MSVC. This is not necessary
because the base mode for MSVC has all features we define except for the
meta-features for C++14 and above. Override the default C++ feature
detection macro for MSVC to run detection only once as before.
Fixes: #17274
Visual Studio 2015 Update 3 introduced the notion of language standard
levels to MSVC. The language standard level is defined in `_MSVC_LANG`
instead of `__cplusplus`. It also added support for the `-std:c++14`
and `-std:c++latest` flags, although the compiler defaults to its C++14
mode anyway. Visual Studio 2017 Update 3 will introduce support for the
`-std:c++17` flag.
Fixes: #16482
There are no specific options for MSVC to set language standards, but
set them as empty strings anyways so the feature test infrastructure
can at least check to see if they are defined.
A common use case of `target_compile_features` is simply to specify that
the compiler should be run in a mode that is aware of e.g. C++11. Some
projects simply specify a particular C++11-only feature to request this.
Provide a first-class way to do this by naming features after the
corresponding language standard. Record them as always available in the
corresponding standard level so that requesting them always ensures that
standard (or higher) is used.
We have no feature tests for versions of VS older than 2010, so do
not even call `record_compiler_features` for such versions. This
is consistent with other compilers where we call this macro only
for versions for which we have recorded features.
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.
