Python Type Hinting: Subclass not recognized.

PyCharm does not acknowledge that an instance of a subclass is also an instance of the superclass. Minimal example:

class A:
    pass

class B(A):
    pass

def function() -> A:
    return B()

b: B = function()

Type Hinting will have a problem at `function()`, claiming that an instance of A is supplied and B is expected.

Production code is slightly more elaborate, with the exact subclass depending on the arguments to the equivalent of `function()`.

It would not be reasonable to expect the type-checker to figure out which subclass is returned, but when explicitly stated that I'm expecting an instance of B which is also an instance of A, I would expect the type check to be satisfied.