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

Comienza con las dos reproducciones autocontenidas del issue y ejecútalas en el mypy playground enlazado usando mypy 1.15.0 y Python 3.12. Compara el tipo inferido devuelto a través de unwrap con el caso de narrowing directo; se considera terminado cuando ambas versiones pasan la comprobación de tipos sin el error de valor de retorno incompatible, conservando la unión declarada.