Generic function type argument inference problem

[JukkaL]

The type checker can't infer the type argument in the call to f below, even though there seems to be no reason why it shouldn't be able to do it:

from typing import List, TypeVar
T = TypeVar('T')
def f(a: List[T], b: List[T]) -> None: pass
x = ['']
f(x, [])  # Cannot infer type argument 1 of "f"

[JukkaL]

After #556 is done, this becomes more annoying.

[rwbarton]

This doesn't seem so easy to fix. We need to either make use of information learned from type checking the first argument to infer a type variable in the second argument (but what if the arguments were in the other order?) or be able to return a polymorphic type when type checking the second argument. I think the latter is easier and it's related to some other changes I want to make anyways (e.g. #1551), but it will require some reorganization.

[gvanrossum]

Based on Reid's comment I'm moving this to 0.5.0 (or maybe even later?).