Feature

Add support for Protocol as TypeIs parameter.

Pitch

Both class types and protocols support isinstance for type narrowing, but TypeIs seems to be not supported for protocols because of mypy error: Narrowed type "Proto" is not a subtype of input type "X".

Adding protocols support in TypeIs would:

• Enable writing general type guard functions without subclassing,
• Provide feature parity between classes (isinstance + TypeIs) and protocols (isinstance only).

Using only isinstance (with @runtime_checkable protocols) is not sufficient, because type guard functions can do more strict runtime checks, and provide TypeIs for static type checkers.

Consider the example below, where hinting X.y as mandatory requires subclassing X instead of declaring independent protocol:

from dataclasses import dataclass
from typing import Protocol, TypeIs, assert_type

@dataclass
class X:
    y: int | None

# Current behavior: have to subclass X

class XHasY(X):
    y: int

def has_y_(obj: X) -> TypeIs[XHasY]:
    return getattr(obj, 'y', None) is not None

x = X(1)
if has_y_(x):
    assert_type(x, XHasY)

# Desired behaviour:

class HasY(Protocol):
    y: int

def has_y(obj: X) -> TypeIs[HasY]:  # mypy error
    return getattr(obj, 'y', None) is not None

x = X(1)
if has_y(x):
    assert_type(x, HasY)

The desired behavior produces mypy error

demo.py:25: error: Narrowed type "HasY" is not a subtype of input type "X"  [narrowed-type-not-subtype]

If mypy error can be easily fixed, this feature could be useful addition to Python type narrowing.