List[Literal[...]] type being narrowed where it shouldn't be

Bug Report

Adding an if check on a specific index of a List of Literals seems to narrow down its scope, even though the list can be modified and the value at that index can change.

To Reproduce

Run mypy over this piece of code:

foo: List[Literal['a', 'b']] = ['a', 'b', 'a']

if foo[0] == 'a':
    print('a')
    exit()

foo.pop(0)
reveal_type(foo[0])
if foo[0] == 'a':
    pass

Expected Behavior

Revealed type to be Literal['a', 'b'], and the conditional to be valid.

Actual Behavior

t.py:11: note: Revealed type is 'Literal['b']'
t.py:12: error: Non-overlapping equality check (left operand type: "Literal['b']", right operand type: "Literal['a']")

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: --strict
• Python version used: 3.9.1 64-bit
• Operating system and version: Fedora 33 x86_64

Comienza ejecutando el reproductor de Python proporcionado con mypy 0.790 e inspecciona el comportamiento del estrechamiento de tipos para Lists mutables indexadas de Literals. Sigue el punto de entrada del estrechamiento para foo[0] a través del checker; se considera terminado cuando reveal_type(foo[0]) sigue siendo Literal['a', 'b'] después de pop(0) y la comprobación de igualdad posterior es aceptada.