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

まず、提供されている Python 再現コードを mypy 0.790 で実行し、インデックス付きの可変 Lists に含まれる Literals の型絞り込みの挙動を調べます。foo[0] の絞り込みのエントリポイントを checker 内で追跡します。完了条件は、pop(0) の後も reveal_type(foo[0]) が Literal['a', 'b'] のままであり、後続の等価性チェックが受け入れられることです。