Allow specifying length of sized object

Sometimes I want to specify the size of a container. This could be done manually with casts, but it would be really cool if Mypy could figure this out automatically. __len__() is guaranteed to return an int, and since a is defined as a literal the match could be statically inferrable. The same is true of other container types.

from typing_extensions import Protocol, Literal


class Char(Protocol):
    def __len__(self) -> Literal[1]:
        ...


a: Char = "X"

Mypy reports

error: Incompatible types in assignment (expression has type "str", variable has type "Char")
note: Following member(s) of "str" have conflicts:
note:     Expected:
note:         def __len__(self) -> Literal[1]
note:     Got:
note:         def __len__(self) -> int

Mypy 0.720
CPython 3.7.3

まず、提供された Python 3.7.3 の例を報告された Mypy 0.720 の動作に対して実行し、プロトコル互換性が len の戻り値の型と Literal 値をどのように処理するかを調べます。コンテナーのサイズを静的に推論できる場合に Char への代入が受け入れられ、他のコンテナー型でも同等の動作になることを完了条件とします。