Feature

If some attribute is not defined for a single union item (often None), we could give a hint that the type may need to be narrowed down.

Pitch

New users don't always figure out that they need an isinstance or an is not None check, and mypy could make this easier for beginners.

Toy example where this could apply:

def f(s: str | None) -> bool:
    # Item "None" of "str | None" has no attribute "startswith"
    return s.startswith('x')

I'm not sure what a good hint could be, but here's an initial idea:

x.py:3: note: You can use "if s is not None" to guard against a None value