Covariance error inferred for lambda has no associated line number

[rkr-at-dbx]

from typing import TypeVar, Generic, Callable

_Tco = TypeVar('_Tco', covariant=True)

class Thing(Generic[_Tco]):

    def chain(self, func):
        # type: (Callable[[_Tco], None]) -> None
        pass

    def end(self):
        # type: () -> None

        return self.chain(lambda _: None)

example.py: error: Cannot use a covariant type variable as a parameter

This error has no associated line number. (If the lambda is expanded into an explicit inner function typed as (_Tco) -> None, the line number is present.)