`TypeIs` with `isinstance` fails to narrow

Bug Report

If I use TypeIs along with isinstance, the type doesn't get narrowed

To Reproduce

from typing import TypeIs, Any, TypeAlias, reveal_type

SliceInt: TypeAlias = slice[int | None, int | None, int | None]
SliceStr: TypeAlias = slice[str | None, str | None, str | None]


def is_slice_int(obj: Any) -> TypeIs[SliceInt]:  # type: ignore[empty-body]
    ...



def main(obj: SliceInt | SliceStr) -> None:
    if is_slice_int(obj):
        pass
    elif isinstance(obj, slice):
        reveal_type(obj)

https://mypy-play.net/?mypy=latest&python=3.14&gist=170b0c83ef93742ff998485be01e9440

Expected Behavior

t.py:19: note: Revealed type is "slice[str | None, str | None, str | None]"

Actual Behavior

t.py:19: note: Revealed type is "slice[int | None, int | None, int | None] | slice[str | None, str | None, str | None]"

Note that, for comparison, pyright and pyrefly get this right:

(scratch) mgorelli@marcoslaptop:~/scratch$ pyright t.py
pyre/home/mgorelli/scratch/t.py
  /home/mgorelli/scratch/t.py:19:21 - information: Type of "obj" is "slice[str | None, str | None, str | None]"
0 errors, 0 warnings, 1 information
(scratch) mgorelli@marcoslaptop:~/scratch$ pyright t.py
/home/mgorelli/scratch/t.py
  /home/mgorelli/scratch/t.py:19:21 - information: Type of "obj" is "slice[str | None, str | None, str | None]"
0 errors, 0 warnings, 1 information
(scratch) mgorelli@marcoslaptop:~/scratch$ pyrefly check t.py
 INFO revealed type: slice[str | None, str | None, str | None] [reveal-type]
  --> t.py:19:20
   |
19 |         reveal_type(obj)
   |                    -----
   |
 INFO 0 errors

Your Environment

• Mypy version used: 2.1.0
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.13.13

リンクされた mypy-play の再現例から始め、TypeIs/isinstance 分岐で明らかになる型を期待される出力と比較します。TypeIs と isinstance の型の絞り込みへのエントリポイントを追跡し、次にその例で明らかになるのが SliceStr だけであることを確認し、該当するテスト箇所が見つかった場合はこのリグレッションのカバレッジを追加します。