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

[finite-state-machine]

Bug Report

Context: mypy doesn't regard Type[object] as Hashable. (It is possible with metaclasses that this might be the case, LSP concerns aside.) It rightly complains when Type[object] is used where a Hashable is required.

The issue is that mypy assumes Type[object] cannot be Hashable, even following an assertion, making the above difficult to work around.

To Reproduce

https://mypy-play.net/?mypy=master&python=3.10&gist=3a8ec03c4472430ecbaeaf397dbc0acd

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

Expected Behavior

Adding assert isinstance(..., Hashable) should work around this problem.

Actual Behavior

Asserting that a variable having type Type[object] is Hashable causes mypy to conclude the code is unreachable.

Your Environment

• Mypy version used: 0.961 and master (on mypy-play.net)
• Mypy command-line flags: (none required; --warn-unreachable adds insight)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.8, 3.10
• Operating system and version: macOS 10.14, mypy-play.net

[AlexWaygood]

Duplicate of #11470

[finite-state-machine]

So it is. My apologies!