Fix overload handling with union types in signatures

[JukkaL]

Previously None was not considered as more precise than
Optional[x], which was obviously incorrect. Fixed the
implementation of type precision checking to match the
description, now that proper subtype checking works.

Fixes the original example in #3295.

[ilevkivskyi]

Why do we need a special case when t is CallableType and s is Instance? It looks like ProperSubtypeVisitor above does exactly the same. So that the whole function boils down to:

    if isinstance(s, AnyType):
        return True
    return is_proper_subtype(t, s)

[gvanrossum]

Good point. I puzzled over this for a while and it really doesn't help that the visitor uses left/right while this function uses t/s.

[JukkaL]

Updated.

[gvanrossum]

I had a similar testcase based on #3295 which still gives an error:

from typing import overload, Optional

@overload
def foo(x: None) -> None:
    pass
@overload
def foo(x: int) -> int:
    pass
def foo(*args):
    pass

def f(a: Optional[int]):
    reveal_type(foo(a))  # Line 13
    if isinstance(a, int):
        reveal_type(foo(a))
    else:
        reveal_type(foo(a))  # Error here

With --strict-optional, the output is unchanged from master:

__tmp__.py:13: error: Revealed type is 'Any'
__tmp__.py:15: error: Revealed type is 'builtins.int'
__tmp__.py:17: error: Revealed type is 'Any'
__tmp__.py:17: error: "foo" does not return a value

[gvanrossum]

@JukkaL This seems stalled waiting for action/response on your end.

[gvanrossum]

@JukkaL, ping?

[JukkaL]

This PR only addresses the first example in #3295. The more general issue requires a much more involved fix, and I think that it can be fixed in a separate PR.

[ilevkivskyi]

This is fine for me as a quick fix. A broader solution for #3295 can be in a separate PR

[JukkaL]

Don't merge yet -- I'll check this with Dropbox internal codebases first.

[JukkaL]

This doesn't work quite right when run against Dropbox internal codebases. I won't merge this until I've resolved the issue.