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

Inizia eseguendo il riproduttore Python fornito con mypy e traccia come vengono inferiti i valori assegnati e le espressioni condizionali durante il controllo degli argomenti Literal. Aggiungi la copertura per le tre chiamate; il lavoro sarà completo quando non produrranno più gli errori di tipo incompatibile segnalati.