Type[object] isn't hashable, and can't be asserted to be Hashable #12993

import collections
import functools
import typing
from typing import (
        Type,
        )


@functools.lru_cache(maxsize=None)
def cached_function(type_: Type[object]) -> bool:
    # expensive computation here
    _ = type_
    return True


def function1(type_: Type[object]) -> None:

    # it's theoretically possible for a Type[...] object to be
    # unhashable (although this would seem to violate the Liskov
    # Substitution Principle, LSP)
    assert cached_function(type_)
            # actual, as expected:
            #       error: Argument 1 to "__call__" of
            #       "_lru_cache_wrapper" has incompatible type
            #       "Type[object]"; expected "Hashable" [arg-type]

def function1a(type_: Type[object]) -> None:

    # we attempt to assure mypy that 'type_' is Hashable
    assert isinstance(type_, typing.Hashable)
    reveal_type(type_)
            # expected: revealed type is: ...
            # actual: prints nothing; mypy believes this is unreachable

    assert cached_function(type_)
            # expected: no warning
            # actual: mypy believes this is unreachable

def function1b(type_: Type[object]) -> None:

    # we attempt to assure mypy that 'type_' is Hashable
    assert isinstance(type_, collections.abc.Hashable)
    reveal_type(type_)
            # expected: revealed type is: ...
            # actual: prints nothing; mypy believes this is unreachable

    assert cached_function(type_)
            # expected: no warning
            # actual: mypy believes this is unreachable