According to cppreference, the gcc libstdc++ supports the parallelism TS. In layperson terms and for what's relevant for me that means #include <execution> works in g++ 9 and doesn't work in g++ 8 or before. In my source code I can handle this with
#if ( defined( __GNUC__ ) && __GNUC__ > 8 )
# define can_use_std_execution
# include <execution>
#endif
For my clang++ builds, the availability of <execution> depends on the --gcc-toolchain that I use. So instead of checking the __clang_major__, I'd like to check the gcc libstdc++ version in the preprocessor.
As far as I see in this compiler-explorer example, __GNUC__ is defined in clang but the compilation command is
-g -o /tmp/compiler-explorer-compiler120120-1672-4ffux6.smufm/output.s -mllvm --x86-asm-syntax=intel -S --gcc-toolchain=/opt/compiler-explorer/gcc-8.3.0 -fcolor-diagnostics -fno-crash-diagnostics /tmp/compiler-explorer-compiler120120-1672-4ffux6.smufm/example.cpp
i.e. the gcc toolchain is from gcc 8.3.0, but the value of __GNUC__ is 4.
What's a good way to query the gcc toolchain version in the preprocessor with clang? Ideally a way that checks the libstdc++ version in a way that's compatible for g++ and clang++ such that I don't have write a spaghetti if that checks the compiler first.