Description

Fixes #9657

Test Plan

I have added three new tests to test-data/unit/check-unions.test

• testNestedProtocolUnions
  This is an exact copy of the MVCE I included in the issue.

• testNestedProtocolGenericUnions
  At first I 'fixed' the issue by not running the for if left is a protocol in mypy.subtypes:is_protocol_implementation. I however noticed that this would still cause the same issue if we changed to Iter = Union[Iterator[T], List[T]]. And all I'd have done is add a subtle bug. This is just the same as testNestedProtocolUnions but to account for non-protocols.

• testNestedProtocolGenericUnionsDeep
  This tests that we can go 5 Iters deep. This is important as we can fix the bug at 3 levels by adding the following to mypy.sametypes:is_same_type.

  if isinstance(left, UnionType) is not isinstance(right, UnionType):
      return False

My changes pass all the tests ran via ./runtests.py.

Origin of the Bug

The issue comes from assuming in mypy.subtypes:is_protocol_implementation.

1. The assumed type is an instance that has a union as its argument.

2. mypy.sametypes:is_same_type works fine for the Instance.

3. We check the Unions are the same type, again with mypy.sametypes:is_same_type.

4. This falls apart as we then call simplify_union which goes on to call mypy.subtypes:is_protocol_implementation.

   In this second call to is_protocol_implementation what we assume the type to be stays the same. This causes problems because we're constantly simplifying what we assume the type to be against itself.

To circumvent this semi-infinite loop I've forced the left value to always be assumed to be a simplified union. This is risky as this is a new assumption. It seems to work, but I'm unsure if this is because of a lack of tests that'd put a non-simplified union on the left side.

Alternates

I can think of two different solutions that don't add an assumption to mypy.sametypes:is_same_type.

1. Adjusting the assumed types in mypy.subtypes:is_protocol_implementation to correctly handle the recursion. I don't know how to implement this.
2. Changing UnionType to have a 'simplified' attribute. I could implement this. But it'd be better wherever the assumptions are being made, and I do not know where that is.