mypy loses information about types. 'str | tuple[str, ...]' becomes 'Sequence[str]'

Bug Report

mypy loses information about types. 'str | tuple[str, ...]' becomes 'Sequence[str]'

To Reproduce

version 1: this fails

from typing import TypeVar

T = TypeVar('T')

def unwrap(x: T | None) -> T:

  assert x is not None

  return x

class Qux:

    def foo(self) -> str | tuple[str, ...]:

        baz = unwrap(self.bar())

        return baz

    def bar(self) -> str | tuple[str, ...] | None:

        return 'quux'

version 2: this works

class Qux:

    def foo(self) -> str | tuple[str, ...]:

        baz = self.bar()

        assert baz is not None

        return baz

    def bar(self) -> str | tuple[str, ...] | None:

        return 'quux'

Expected Behavior

both versions of code should run successfully

Actual Behavior

version 2 fails with the following error: 'Incompatible return value type (got "Sequence[str]", expected "str | tuple[str, ...]") [return-value]'

Your Environment

mypy version: 1.15.0
python version: 3.12

reproducible even the mypy playground

Start with the two self-contained reproductions in the issue and run them in the linked mypy playground using mypy 1.15.0 and Python 3.12. Compare the inferred type returned through unwrap with the direct narrowing case; done means both versions type-check without the incompatible return-value error while preserving the declared union.