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

Start by running the supplied Python 3.7.3 example against the reported Mypy 0.720 behavior and examine how protocol compatibility handles len return types and Literal values. Done means the assignment to Char is accepted when the container size is statically inferable, with equivalent behavior for other container types.