Inference for Literal arguments not handled

Bug Report

If you have a function with Literal arguments, and a variable that is set to one of the options, an error is reported.

To Reproduce

from typing import Literal


def myfun(a: Literal["a", "b", "c"]) -> str:
    return a


s = "a"

myfun(s)


def myfun2(a: Literal[1, 2]) -> int:
    return a


b = 2
myfun2(b)

c = 1 if s == "a" else 2
myfun2(c)

reports

literalinf.py:10: error: Argument 1 to "myfun" has incompatible type "str"; expected "Union[Literal['a'], Literal['b'], Literal['c']]"
literalinf.py:18: error: Argument 1 to "myfun2" has incompatible type "int"; expected "Union[Literal[1], Literal[2]]"
literalinf.py:21: error: Argument 1 to "myfun2" has incompatible type "int"; expected "Union[Literal[1], Literal[2]]"

Expected Behavior

No error. The values can be inferred to be of the Literal value

Actual Behavior

See above.

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.5
• Operating system and version: Windows 10 21H1

まず、提供された Python 再現コードを mypy で実行し、Literal 引数をチェックするときに代入された値と条件式がどのように推論されるかを追跡します。3 つの呼び出しをカバーするテストを追加し、報告された互換性のない型エラーがそれらで発生しなくなった時点で作業完了とします。