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

Start by running the supplied Python reproducer with mypy and trace how assigned values and conditional expressions are inferred when checking Literal arguments. Add coverage for the three calls, with the work complete when they no longer produce the reported incompatible-type errors.