These are new features of C2x, but they're supported as a conforming extension in older versions of Clang. However, there's no pedantic warning given about them being a Clang extension in older language modes, so users may be surprised when porting to another compiler. See https://godbolt.org/z/MdK476q7b as an example. Should we be issuing a pedantic warning here?