Description

Running ruff check --select=PYI019 --preview foo.pyi triggers a panic if foo.pyi has this contents:

class F:
    @classmethod
    def m[S](cls: type[S], /) -> S[int]: ...

The panic occurs at this line here:

And the reason is that, contrary to what the comment says, uses_custom_typevar does not guarantee that the return Expr will be an ExprName, due to the map_subscript call here:

(The map_subscript call also looks buggy, FWIW.)

We should switch to a more type-safe code pattern here. Note that although the panic occurs in autofix logic, the panic occurs even if you run Ruff without --fix, since we create fixes for diagnostics even if --fix is not specified.