`TypeIs[x]` should be treated as a subtype of `bool`

[erictraut]

Pyright does not currently perform proper subtyping checks for TypeIs and TypeGuard. In particular, it treats TypeIs[x] as equivalent to bool when performing subtyping checks. TypeIs[x] should be considered a subtype of bool, and TypeIs[x] should be considered a subtype of TypeIs[y] only if x is a subtype of y.

This can be demonstrated in this code, where the Any input to is_dataclass should cause the overload logic to resolve both overloads rather than the first.

import dataclasses
from typing import reveal_type
def func(x: Any) -> None:
    if dataclasses.is_dataclass(x) and isinstance(x, object):
        reveal_type(x)
        print(dataclasses.asdict(x))

This is related to this discussion.

[InSyncWithFoo]

I think I just came across this as well (playground):

from typing import Literal, TypeIs, overload

@overload
def f(v: tuple[int | str, ...], b: Literal[False]) -> TypeIs[tuple[str, ...]]: ...

@overload
def f(v: tuple[int | str, ...], b: Literal[True] = True) -> TypeIs[tuple[int, ...]]: ...

# Function return type "TypeIs[tuple[str, ...]]" is incompatible with type "bool"
def f(v: tuple[int | str, ...], b: bool = True) -> bool: ...