Can't narrow `Sequence[<X>]` to `Tuple[<X>, ...]` using `isinstance(..., tuple)`

[finite-state-machine]

Bug Report

Using isinstance(<Sequence[X]>, tuple) narrows the type of <Sequence[X]> to Tuple[Any, ...], not Tuple[X, ...] as one would expect.

To Reproduce

[mypy-play.net]
[pyright-play.net¹] (pyright handles this as expected)

from __future__ import annotations
from typing_extensions import (
        assert_type,
        Sequence,
        Tuple,
        )

value: Sequence[str]

if isinstance(value, tuple):
    assert_type(value, Tuple[str, ...])
            # expected: (this works)
            # actual: "Expression is of type 'tuple[Any, ...]',
            #          not 'tuple[str, ....]'"

Expected Behavior

if instance(value, ...) should only ever narrow the type of value

Actual Behavior

Mypy narrows the container type, but forgets the element type

Your Environment

• Mypy version used: 1.8.0; bug exists back to at least 1.0
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): (none)
• Python version used: 3.8, 3.12

Existing issues

While it seems extremely unlikely that this issue hasn't been reported, I haven't been able to locate anything obviously related among the ~217 issues that mention both isinstance and tuple. Issue #2456 may be vaguely related.

Footnotes

1. the pyright example code is slightly modified (1) to avoid an error around value being uninitialized, and (2) to reflect that pyright behaves as expected ↩