Two more issues with overloads and --no-strict-optional

[ilevkivskyi]

Here are some new false positives with overloads, based on real code samples with map in Python 2.
The bug(s) only appear if the strict optional is disabled:

from typing import TypeVar, Iterable, Tuple, List, overload, Callable
S = TypeVar('S')
T1 = TypeVar('T1')
T2 = TypeVar('T2')

@overload
def map1(func: Callable[[T1], S], iter1: Iterable[T1]) -> List[S]: ...
@overload
def map1(func: None, iter1: Iterable[T1]) -> List[T1]: ...
def map1(*args, **kwargs):
    pass

a: List[int]
b: List[str]

for x in map1(None, a):  # false positive: Need type annotation for 'x'
    pass

@overload
def map2(func: Callable[[T1, T2], S],
        iter1: Iterable[T1],
        iter2: Iterable[T2]) -> List[S]: ...
@overload
def map2(func: None,
        iter1: Iterable[T1],
        iter2: Iterable[T2]) -> List[Tuple[T1, T2]]: ...
def map2(*args, **kwargs):
    pass

for y, z in map2(None, a, b):  # false positive: '<nothing>' object is not iterable
    pass

[Michael0x2a]

A temporary workaround would be to swap the order of the overloads so the one with func: None always comes first.

I'll look into adding a more robust fix.