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.