Fix false positive unreachable error for isinstance(x, UnionType) on TypeForm

[Dextheking1]

Bug

With --warn-unreachable, mypy reported Statement is unreachable [unreachable] for isinstance(t, UnionType) when t: TypeForm[Any]:

from types import UnionType
from typing import Any
from typing_extensions import TypeForm

def g(t: TypeForm[Any]) -> Any:
    if isinstance(t, UnionType):  # error: Statement is unreachable [unreachable]
        pass

This is a false positive: at runtime a type expression like int | str is an instance of types.UnionType, so the check can genuinely succeed. mypy treated TypeForm and types.UnionType as disjoint.

(The sibling behavior for isinstance(t, Union) with bare Union is untouched — bare Union still errors with arg-type since it can't be used in isinstance at runtime.)

Fix

mypy/meet.py: teach is_overlapping_types that a TypeForm[T] overlaps with types.UnionType when the item can be a union expression — i.e. Any, a union, or an unconstrained type variable (via new helper _type_form_item_can_be_union). The check runs before the existing Type[C] vs metaclass case, which would otherwise return False.

Precision is preserved:

• TypeForm[int] vs UnionType is still correctly reported unreachable (int is never a UnionType instance)
• plain type[Any] (non-TypeForm) behavior is unchanged
• the yes-branch narrows to types.UnionType; the else-branch keeps TypeForm[Any]

Test evidence

New regression test testTypeFormIsinstanceUnionType in test-data/unit/check-typeform.test (flags: --warn-unreachable) covering TypeForm[Any], TypeForm[int | str] (reachable, narrows to types.UnionType) and TypeForm[int] (still unreachable).

• New test fails before the fix (Statement is unreachable on the reachable branches) and passes after
• Suites run, all green: check-typeform.test + check-isinstance.test (244 passed), check-narrowing.test (175 passed, 1 xfailed), check-python310.test (202 passed, 1 skipped), check-overloading.test (272 passed, 1 skipped), testsubtypes.py (26 passed)

Closes #21910.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅