It's a common extension but it's not standard. Both C23 and C++23 only allow identifiers to start with a Unicode XID_START character, which excludes $. And with older standards, it was just Latin letters, the underscore, and explicit \u escapes.

If you look in the project's pedantic.cmake, you'll find a add_c_and_cxx_compile_options("-Wno-dollar-in-identifier-extension") just to suppress warnings about this.