The compiler attr in commands is weird, it's never set in finalize_options, but rather in run instead. (normally we treat anything in finalize_options in setuptools the same as if it's been initialized in __init__)
With downstream command subclasses also intended to override finalize_options, this means compiler may not be an actual Compiler class instance yet.

Tentatively opening to see ecosystem check.

This would, once again, benefit from AnyOf: python/typing#566