I recommend omitting the default case and putting std::unreachable() outside the switch and omitting the default label for the sole reason that compilers are more likely to warn for a missed case label this way (-Wswitch vs -Wswitch-enum in gcc/clang, the former is included in -Wall, the latter isn't included even in -Wextra).

This also allows expressing intent: no default label means that I meant to handle all cases, and having a default means that I opted into a fallback, please don't warn. That's probably why -Wswitch-enum isn't enabled by default, too many false positives without a convenient way to suppress the warning.