Type narrowing discards tuple from `Union[Tuple, Sequence]` in conditional

[zigaLuksic]

Bug Report

When refining an element of type Union[Tuple, Sequence] with the isinstance(x, Sequence) in a conditional, the Tuple part of the type is wrongly discarded in the branch.

To Reproduce

from typing import Sequence, Tuple, Union


def test_func(x: Union[Tuple[int], Sequence[str]]) -> str:
    if isinstance(x, Sequence):
        reveal_type(x)  # Revealed type is "typing.Sequence[builtins.str]"
        return x[0]  # raises no error
    return "a"  # Statement is unreachable

Expected Behavior

The revealed type of x should be Union[Tuple[int], Sequence[str]] or something equivalent. The line return x[0] should be marked as an issue, because currently test_func((1,)) returns 1 so the functions shouldn't typecheck.

Actual Behavior

test.py:6:21: note: Revealed type is "typing.Sequence[builtins.str]"
test.py:8:5: error: Statement is unreachable  [unreachable]
Found 1 error in 1 file (checked 1 source file)

Your Environment

Used a fresh virtual environment

• Mypy version used: 0.971
• Python version used: 3.8

[brianschubert]

This was fixed in #16470, released with v1.8.0. Mypy now correctly infers a union type inside the if branch:

from typing import Sequence, Tuple, Union

def test_func(x: Union[Tuple[int], Sequence[str]]) -> str:
    if isinstance(x, Sequence):
        reveal_type(x)  # N: Revealed type is "Union[tuple[builtins.int], typing.Sequence[builtins.str]]"
        return x[0]  # E: Incompatible return value type (got "int | str", expected "str")
    return "a"  # Statement is unreachable

Thanks for the original report!