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

Start by running the provided Python reproducer with mypy 0.790 and inspect the type-narrowing behavior for indexed mutable Lists of Literals. Trace the narrowing entry point for foo[0] through the checker; done means reveal_type(foo[0]) remains Literal['a', 'b'] after pop(0), and the later equality check is accepted.