Context causes invalid type argument value to be inferred

[JukkaL]

The type checker may use context to infer an invalid type variable value for a type variable such as AnyStr:

from typing import AnyStr

def f(x: AnyStr) -> AnyStr:
    if isinstance(x, str):
        return 'foo'
    else:
        return b'zar'

print(repr(f(''))) # Type argument 1 of "f" has incompatible value "object"

We should probably not use type context in cases like this when the type variable is not used within a generic type.

[spkersten]

The type variable AnyStr is restricted to str or bytes. So, it seems to me that the problem here is that the return type of f is inferred to be object, which is wider than AnyStr.

I think the context should be used to restrict the possible values of the type var, instead of replacing it like seems to happen now.

So in the above example, repr expects an object, so that would not restrict AnyStr. However, in this continuation of the example:

def g(s: str): pass

g(f(''))

The context (str) would be used to restrict (and uniquely determine in this case) AnyStr to str.

I'll try to implement this.