What you're asking for would essentially require attributes to become a
part of a function's signature. I don't think this is a door that should be
opened.

As Giuseppe stated, the warning message can be turned off by annotating the
calling function's implementation (which you already statically stated is
[[noreturn]]) with [[unreachable]]. It's annoying perhaps, but given that
these are very isolated corner cases, the motivation for an actual language
change (particularly relating to function types) is pretty minimal.