Wrong type hint after unpacking/swapping

Bug Report

Unpacking a tuple in the below scenario results in wrong assumed types.

To Reproduce

import typing as t

def spam(val: t.Union[t.Type, None] = None) -> None:

  def eggs(type_: t.Type) -> None:
    pass

  reveal_type(val)
  if isinstance(val, type):
    reveal_type(val)
    val, foo = None, val
    reveal_type(val)
    reveal_type(foo)
    return eggs(foo)

Expected Behavior

After the line val, foo = None, val, I would expect that

1. val has type None (ok)
2. foo has type Type[Any]
3. the call to eggs() passes

Actual Behavior

$ mypy test.py 
test.py:9: note: Revealed type is 'Union[Type[Any], None]'
test.py:11: note: Revealed type is 'Type[Any]'
test.py:13: note: Revealed type is 'None'
test.py:14: note: Revealed type is 'None'
test.py:15: error: Argument 1 to "eggs" has incompatible type "None"; expected "Type[Any]"
Found 1 error in 1 file (checked 1 source file)

This does not match with my expectations 2 and 3.

Your Environment

Python 3.9
Mypy 0.790
MacOS 10.14.6

Commencez par exécuter avec mypy l’exemple Python signalé et comparez les types révélés avant et après val, foo = None, val. Étudiez la gestion par le vérificateur de types du déballage de tuples et des variables dont le type a été affiné. Le travail est terminé lorsque foo reste Type[Any] et que l’appel à eggs(foo) passe sans erreur.