Dict in does not narrow down Union

[james10424]

Bug Report

(A clear and concise description of what the bug is.)

To Reproduce

from typing import Dict, Union, NewType

A = NewType("A", str)
B = NewType("B", str)

dict_a: Dict[A, str] = {}

def f(a: Union[A, B]):
    if a in dict_a:
        dict_a[a]

Expected Behavior

Mypy recognizes that a inside the if statement is type A, because dict_a's keys are of type A and a is one of the keys.

Actual Behavior

error: Invalid index type "Union[A, B]" for "Dict[A, str]"; expected type "A"

Your Environment

• Mypy version used: v1.1.1
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10

[AlexWaygood]

Duplicate of #10977