[Python] Negative narrowing of union type with TypeIs of TypeVar not working as expected in...

I recently upgraded mypy from 1.17.0 to 1.18.2 The following code was successfully validated in the old mypy version (1.17.0), but fails in the new one (1.18.2):

_T = TypeVar('_T')


class Foo(Generic[_T], ABC):
pass


def to_foo(value: _T | Foo[_T],
checker: Callable[[Any], TypeIs[_T]],
factory: Callable[[_T], Foo[_T]]) -> Foo[_T]:
if checker(value):
return factory(value)
else:
return value


This is the error:

error: Incompatible return value type (got "_T | Foo[_T]", expected "Foo[_T]") [return-value]


mypy seems unhappy with the second return statement. It seems, it can not deduce, that if value is _T | Foo[_T] to begin with, and is not _T (the else case), that is has to be of type Foo[_T].

Although, I thought that is exactly what TypeIs does:

Type narrowing is applied in both the positive and negative case
​

When I reduce the problem further, I end up with the following function which still fails:

_T = TypeVar('_T')


def mypy_fails(x: _T | int, checker: Callable[[Any], TypeIs[_T]], default: int) -> int:
if checker(x):
return default
else:
return x


It seems to be related to the TypeVar, since the following function (with int and _T switched) still validates:

def mypy_validates(x: int | _T, checker: Callable[[Any], TypeIs[int]], default: _T) -> _T:
if checker(x):
return default
else:
return x


What is wrong with my initial code? Why does mypy_validates work? And why was it fine in the old mypy version, but not the newer one?

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