A type can be consistent subtype of distinct materializations of a generic protocol

Bug Report

When using TypeIs to narrow a type down to two different fully static materializations of the same generic Protocol, MyPy will infer the result to be Never, thus producing false assertions.

To Reproduce

from typing import Any, Literal, Protocol, TypeIs

# ==== Basic definitions ====

class MockProtocol[T](Protocol):  # T is inferred to be contravariant
    def __call__(self, t: T) -> None:
        ...

class MockClass():
    def __call__(self, t: int | str) -> None:  # Implements MockProtocol[int | str]
        pass

# ==== Basic static type checking ====

def only_accept_mock_int(obj: MockProtocol[int]) -> None:
    pass

def only_accept_mock_str(obj: MockProtocol[str]) -> None:
    pass

only_accept_mock_int(MockClass())  # MockClass is subtype of MockProtocol[int]
only_accept_mock_str(MockClass())  # MockClass is subtype of MockProtocol[str]

# ==== Runtime type checking involving TypeIs ====

def is_mock_int(obj: Any) -> TypeIs[MockProtocol[int]]:
    return isinstance(obj, MockClass)  # Mock implementation

def is_mock_str(obj: Any) -> TypeIs[MockProtocol[str]]:
    return isinstance(obj, MockClass)  # Mock implementation

def check1(obj: MockClass) -> Literal[True]:  # Won't complain because `return False` is unreachable
    if is_mock_str(obj) and is_mock_int(obj):
        return True
    return False  # Inferred to be unreachable

def check2(obj: MockProtocol[str]) -> Literal[False]:  # Won't complain because `return True` is unreachable, which is incorrect!
    if is_mock_str(obj) and is_mock_int(obj):
        return True  # Inferred to be unreachable
    return False

result1 = check1(MockClass())  # Inferred to be Literal[True]
result2 = check2(MockClass())  # Inferred to be Literal[False], but the value is True!!!
assert result1 == result2  # Inferred to fail consistently, but it should succeed

Expected Behavior

check2 should be inferred as Callable[..., bool] because a fully static type can be subtype of both MockProtocol[str] and MockProtocol[int].

Actual Behavior

Literal[False] is recognized as a compatible return type of check2 due to false reachability assumption, causing the type of result2 to be wrong.

Your Environment

• Mypy version used: 1.18.1
• Mypy command-line flags: mypy --strict
• Mypy configuration options from mypy.ini (and other config files): (default)
• Python version used: Python 3.13.8

Chạy bản tái hiện được cung cấp với mypy --strict và kiểm tra cách việc narrowing của TypeIs xử lý hai materialization generic của Protocol. Thêm một regression test bao phủ check1 và check2, sau đó xác minh rằng các kiểu trả về được suy luận và hành vi của assertion khớp với các kết quả mong đợi.