Eager narrowing to `Literal` when possible

Feature

I often feel I have to put too much effort into informing mypy that some variables can only hold a finite set of values. It would be helpful if mypy was a bit smarter in the way it infers Literal types.

Pitch

Here are some concrete examples:

from typing import Literal

def letter_v1(number: int) -> Literal["a", "b", "c"]:
    if number == 1:
        letter = "a"
    elif number == 2:
        letter = "b"
    else:
        letter = "c"
    
    # Incompatible return value type (got "str", expected "Literal['a', 'b', 'c']")  [return-value]
    return letter
    
def letter_v2(number: int) -> Literal["a", "b", "c"]:
    letter = "a" if number == 1 else "b" if number == 2 else "c"
    
    # Incompatible return value type (got "str", expected "Literal['a', 'b', 'c']")  [return-value]
    return letter

def letter_v3(number: int) -> Literal["a", "b", "c"]:
    letter = ("a", "b", "c")[number]
    
    # Incompatible return value type (got "str", expected "Literal['a', 'b', 'c']")  [return-value]
    return letter

See also: https://mypy-play.net/?mypy=1.3.0&python=3.11&flags=strict&gist=bab569e05877621b90f081fc7bdb7d05

Ideally, I think I should get away without having to specify the type of letter in any of the concrete examples here. A human reviewing this code would easily conclude that there is no issue here, yet mypy struggles.

リンクされた mypy-play のケースを使って、3 つの letter_v1、letter_v2、letter_v3 の例を再現し、その後、mypy がローカル変数の型をどのように推論するかを追跡します。意図された narrowing の動作を特定し、これらの例の回帰テストカバレッジを追加します。letter に明示的な型を指定しなくても、注釈付き関数が型チェックを通れば完了です。