Overload with Dict only checks overlap on keys

[rhshadrach]

Using mypy 0.782, it looks like overlap is only checked on dictionary keys.

from typing import Dict, overload

class A: pass
class B: pass

@overload
def foo(x: Dict[A, A]) -> A: ...

@overload
def foo(x: Dict[A, B]) -> B: ...

def foo(x):
    for k, v in x: return v
    raise ValueError

gives error: Overloaded function signatures 1 and 2 overlap with incompatible return types. I get the same error when using Mapping instead of `Dict.

[gvanrossum]

It's not specific to Dict or Mapping, it seems to happen with any generic class over two type variables.

I do not actually have a good theory as to why it happens, but I suspect there is a case where a type can be either a Dict[A, A] or a Dict[A, B]. Perhaps the possibility of multiple inheritance is key:

class C(A, B): ...
a: Dict[A, C] {}
foo(a)

[rhshadrach]

Thanks for the response. If I change the 2nd overload to Dict[B, B], then mypy doesn't give any errors. I think the ambiguity with multiple inheritance still exists in this case (e.g. Dict[C, C]), yes?

[JelleZijlstra]

I suspect it's this code: https://github.com/python/mypy/blob/master/mypy/meet.py#L342. is_overlapping_types() returns True if any of the type arguments are overlapping, but presumably it should only do that if they all overlap.