Incorrect `TypeIs` narrowing for tuples

[erictraut]

I noticed this behavior when investigating a similar issue in pyright.

Mypy isn't correctly narrowing tuples in some cases.

from typing_extensions import TypeIs


def is_tuple_of_strings(v: tuple[int | str, ...]) -> TypeIs[tuple[str, ...]]:
    return all(isinstance(x, str) for x in v)


def test1(t: tuple[int]) -> None:
    if is_tuple_of_strings(t):
        reveal_type(t)  # Should be Never ✅ 
    else:
        reveal_type(t)  # Should be tuple[int] ✅ 

def test2(t: tuple[str, int]) -> None:
    if is_tuple_of_strings(t):
        reveal_type(t)  # Should be Never ✅ 
    else:
        reveal_type(t)  # Should be tuple[str, int] ✅ 

def test3(t: tuple[int | str]) -> None:
    if is_tuple_of_strings(t):
        reveal_type(t)  # Should be tuple[str] ✅ 
    else:
        reveal_type(t)  # Should be tuple[int] or tuple[int | str] ❌ (mypy: Never)

def test4(t: tuple[int | str, int | str]) -> None:
    if is_tuple_of_strings(t):
        reveal_type(t)  # Should be tuple[str, str] ✅ 
    else:
        reveal_type(t)  # Should be tuple[int | str, int | str] or tuple[int, int | str] | tuple[str, int] ❌ (mypy: Never)

def test5(t: tuple[int | str, ...]) -> None:
    if is_tuple_of_strings(t):
        reveal_type(t)  # Should be tuple[str, ...] ✅ 
    else:
        reveal_type(t)  # Should be tuple[int | str, ...] ❌ (mypy: Never)

def test6(t: tuple[str, *tuple[int | str, ...], str]) -> None:
    if is_tuple_of_strings(t):
        reveal_type(t)  # Should be tuple[str, *tuple[str, ...], str] ❌ (mypy: tuple[str, Never, str])
    else:
        reveal_type(t)  # Should be tuple[str, *tuple[int | str, ...], str] ❌ (mypy: Never)

[kreathon]

I just checked the outcome of the tests with: https://github.com/python/mypy/pull/17232:

• 3,4,5 would also be fixed
• 6 does not change (at least the if-branch; else branch looks correct)

Output:

../xx.py:12: note: Revealed type is "tuple[builtins.int]"
../xx.py:18: note: Revealed type is "tuple[builtins.str, builtins.int]"
../xx.py:22: note: Revealed type is "tuple[builtins.str]"
../xx.py:24: note: Revealed type is "tuple[Union[builtins.int, builtins.str]]"
../xx.py:28: note: Revealed type is "tuple[builtins.str, builtins.str]"
../xx.py:30: note: Revealed type is "tuple[Union[builtins.int, builtins.str], Union[builtins.int, builtins.str]]"
../xx.py:34: note: Revealed type is "builtins.tuple[builtins.str, ...]"
../xx.py:36: note: Revealed type is "builtins.tuple[Union[builtins.int, builtins.str], ...]"
../xx.py:40: note: Revealed type is "tuple[builtins.str, Never, builtins.str]"
../xx.py:42: note: Revealed type is "tuple[builtins.str, Unpack[builtins.tuple[Union[builtins.int, builtins.str], ...]], builtins.str]"

[hauntsaninja]

This is mostly fixed by https://github.com/python/mypy/pull/18193 There is still a variadic case that is unfixed