`type[T]` cannot be an instance of a runtime checkable protocol

[pulustelu]

Describe the bug
isinstance checks comparing instances of type / type[object] / type[T] / type[Any] to any @runtime_checkable marked Protocol infer the instance as Never.

Expected behavior
The instance should be inferred as the intersection of type and the given protocol, i.e. <subclass of type and ...>.

Screenshots or Code
If applicable, add screenshots or the text of the code (surrounded by triple back ticks) to help explain your problem.

from typing import Any, Protocol, runtime_checkable

@runtime_checkable
class Foo(Protocol):
    foo: int # or ClassVar[int]

def bar(cls: type[Any]): # or any other type expression
    if isinstance(cls, Foo):
        reveal_type(cls) # Never

VS Code extension or command-line

• Pylance v2023.1.20

Additional context

Mypy infers the reveal_type call to be unreachable. However, if type[Any] is replaced with type, the inferred result is __main__.<subclass of "type" and "Foo">. Evidently, since these behaviors are different there is an additional bug on mypy's end here too.

[erictraut]

The behavior of isinstance narrowing is not well defined. Pyright's implementation has been tuned over time to produce the "best" answer most of the time. I don't think that producing an intersection in this case is the best approach.

If you change your annotation for cls to be either object or Any, it will be narrowed to Foo in your example.

The type of type and type[Any] should be equivalent, so mypy's behavior is inconsistent.