False positive "Exception type must be derived from BaseException"

[JelleZijlstra]

Mypy incorrectly emits an error when using a variable typed as a variadic tuple of exception classes in an except clause. fn1 and fn2 below trigger an error:

from typing import Tuple, Type, Union

def fn1(exc: Tuple[Type[Exception], ...]) -> None:
 try:
 1 / 0
 except exc: # E: Exception type must be derived from BaseException
 pass

def fn2(exc: Union[Type[Exception], Tuple[Type[Exception], ...]]) -> None:
 try:
 1 / 0
 except exc: # E: Exception type must be derived from BaseException
 pass

def fn3(exc: Type[Exception]) -> None:
 try:
 1 / 0
 except exc: # this is fine
 pass

def fn4(exc1: Type[Exception], exc2: Type[Exception]) -> None:
 try:
 1 / 0
 except (exc1, exc2): # this is fine too
 pass

def fn5(exc: Tuple[Type[Exception], Type[Exception]]) -> None:
 try:
 1 / 0
 except exc: # no error
 pass

I came across this trying to add types to a function similar to @retry() from https://github.com/quora/qcore/blob/master/qcore/decorators.py#L204.

[gvanrossum]

Good find. Looks like these cases can all easily be added to visit_except_handler_test() in checker.py. (The variadic tuple case should go in the initialization of test_types, the union case should go in the if/elif sequence.)