TypeGuard is assignable to bool

[layday]

PEP 647, under the heading "TypeGuard Type", says:

In all other respects, TypeGuard is a distinct type from bool. It is not a subtype of bool. Therefore, Callable[..., TypeGuard[int]] is not assignable to Callable[..., bool].

However, 1. type checks both in Pyright and mypy, and both seem to treat TypeGuard as a subtype of bool. This means that the overload order in 2. is significant:

# pyright: strict

from typing import Callable, Iterable, TypeVar, overload
from typing_extensions import TypeGuard


# 1.

def foo(guard: Callable[..., bool]) -> None:
    ...

def bar(value: object) -> TypeGuard[int]:
    return True


foo(bar)

baz: Callable[..., bool] = bar


# 2.

_T = TypeVar('_T')
_S = TypeVar('_S')

@overload
def qux(guard: Callable[[_T], bool], value: Iterable[_T]) -> Iterable[object]: ...
@overload
def qux(guard: Callable[[_T], TypeGuard[_S]], value: Iterable[_T]) -> Iterable[_S]: ...

quux = list(qux(bar, ['a']))
reveal_type(quux)  # Type of "quux" is "list[object]"

Am I misinterpreting the PEP, is this uninentional, or is it intentional and does the PEP need updating?

[erictraut]

You are misinterpreting the PEP. You would need to swap the two overloads to make this work. The TypeGuard should come first.