The grammar will 'work' without the predicate as well, but will then allow =1, =2, =3, =4, =5, =6 and =7 as alternative pure virtual function specifiers in this case. Sorry if I made any specific ...
Various software that used to build fine on older versions of macOS (with its older libc++) when using a newer version of llvm.org clang now fails to build with llvm.org clang 16 and 17 with a message ...