`TypeIs` narrowing of union type in a generic is not working

Bug Report

Given an object of type int | bytes and a TypeIs function that narrows int | str | bytes into int | str, mypy correctly infers that the original object is really just int.

However, this doesn't work if the union is a generic parameter. That is, given Sequence[int | bytes] and a narrowing function from Sequence[int | str | bytes] to Sequence[int | str], mypy doesn't infer that the original is Sequence[int]. Instead, it infers that the original is Sequence[int | str].

I think this is a bug? But of course I might just be holding it wrong. Any help will be greatly appreciated.

To Reproduce

from typing import Callable, TypeIs, Sequence, assert_type

def check_plain(value: int | str | bytes) -> TypeIs[int | str]:
    return isinstance(value, int | str)

def check_sequence(seq: Sequence[int | str | bytes]) -> TypeIs[Sequence[int | str]]:
    return all(isinstance(value, int | str) for value in seq)

def works(value: int | bytes) -> None:
    assert check_plain(value)
    assert_type(value, int)

def doesnt(value: Sequence[int | bytes]) -> None:
    assert check_sequence(value)
    assert_type(value, Sequence[int])

https://mypy-play.net/?mypy=1.19.0&python=3.13&gist=100053f110206f3fdf37170ea75a3ca2

Actual Behavior

main.py:15: error: Expression is of type "Sequence[int | str]", not "Sequence[int]"  [assert-type]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.19.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.13

まず、提供された再現コードを mypy 1.19.0 で実行し、汎用的な Sequence のケースで失敗する assert_type を確認します。ジェネリックパラメーターに対する TypeIs の narrowing ロジックを追跡します。完了条件は、報告されたエラーなしに例で value が Sequence[int] に絞り込まれ、一方で単純なケースが引き続き動作することです。