Apple nowadays mainly focus on Swift, and C++ support only needs to be good enough for Metal Shading Language (a C++14 dialect), IO and DriverKit needs, and compiling LLVM (currently requires ISO C++17).
Likewise, on Google's side, those that went on to work on Carbon are no longer contributing to clang.
All the other compiler vendors that have clang forks, seem more interested into LLVM improvements than ISO C++ compliance, thus now clang lags behind GCC and VC++ in ISO C++ capabilities.
Apple nowadays mainly focus on Swift, and C++ support only needs to be good enough for Metal Shading Language (a C++14 dialect), IO and DriverKit needs, and compiling LLVM (currently requires ISO C++17).
Likewise, on Google's side, those that went on to work on Carbon are no longer contributing to clang.
All the other compiler vendors that have clang forks, seem more interested into LLVM improvements than ISO C++ compliance, thus now clang lags behind GCC and VC++ in ISO C++ capabilities.