Mypy doesn't recognize that different string literals in if/else statement match a union of string literals

Bug Report

When a function has a type signature like def foo(bar: Literal["a", "b"]) -> None: ... mypy throws an error if the argument passed in can be either a or b as it implies a general string.

To Reproduce

from typing import Literal

CHOICE = bool(...)

def foo(bar: Literal["a", "b"]) -> None: ...

def main() -> None:
    bar = "a"
    if CHOICE:
        bar = "b"

    foo(bar)

Expected Behavior

It would be nice if mypy recognized that bar here could only be Literal["a"] or Literal["b"], i.e Literal["a", "b"].

Actual Behavior

mre.py:12: error: Argument 1 to "foo" has incompatible type "str"; expected "Literal['a', 'b']" [arg-type]

Your Environment

• Mypy version used: 1.14.1
• Mypy command-line flags: Default
• Mypy configuration options from mypy.ini (and other config files): Default
• Python version used: 3.13

Context

This came up because of a recent change in typeshed https://github.com/python/typeshed/pull/12181 which causes this pip code to trigger a mypy error: https://github.com/pypa/pip/blob/24.3.1/src/pip/_internal/utils/unpacking.py#L179

I assume this issue of different implicit Literal / str types inside branches has been discussed before but I couldn't find it searching through the GitHub issues.

x-ref: https://github.com/pypa/pip/pull/13148

Riproduci la diagnostica in mre.py con mypy 1.14.1 usando l’esempio mostrato di assegnazione in un branch, quindi traccia il modo in cui il type checker inferisce bar nei percorsi if/else. Usa il riferimento pip unpacking.py come caso del mondo reale; il lavoro è completato quando entrambi gli esempi accettano l’unione di string literal senza un errore incompatible-type.