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Unclear when Join is picked vs Union in upcast operation. #19652
Description
I noticed some rather erratic behavior when sometimes it picks union and other times the join.
from typing import Iterable
class A: ...
class B: ...
class C: ...
def upcast[T](arg: Iterable[T]) -> Iterable[T]: return arg
def test_builtin(x: tuple[int, str, None]) -> None:
reveal_type(upcast(x)) # Iterable[str | int | None]
def test_custom(x: tuple[A, B, C]) -> None:
reveal_type(upcast(x)) # Iterable[object]
def test_mixed1(x: tuple[A, str, None]) -> None:
reveal_type(upcast(x)) # Iterable[str | A | None]
def test_mixed2(x: tuple[int, str, C]) -> None:
reveal_type(upcast(x)) # Iterable[object]https://mypy-play.net/?mypy=latest&python=3.12&gist=833674a36174c4140a89645b609da8a9
Activity
randolf-scholz commented on Aug 13, 2025
It seems to depend on whether None is one of the tuple elements.