René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Unclear when Join is picked vs Union in upcast operation.","articleBody":"I noticed some rather erratic behavior when sometimes it picks union and other times the join.\n\n```python\nfrom typing import Iterable\n\nclass A: ...\nclass B: ...\nclass C: ...\n\ndef upcast[T](arg: Iterable[T]) -\u003e Iterable[T]: return arg\n\ndef test_builtin(x: tuple[int, str, None]) -\u003e None:\n    reveal_type(upcast(x))  # Iterable[str | int | None]\n\ndef test_custom(x: tuple[A, B, C]) -\u003e None:\n    reveal_type(upcast(x))  # Iterable[object]\n\ndef test_mixed1(x: tuple[A, str, None]) -\u003e None:\n    reveal_type(upcast(x))  # Iterable[str | A | None]\n\ndef test_mixed2(x: tuple[int, str, C]) -\u003e None:\n    reveal_type(upcast(x))  # Iterable[object]\n```\n\nhttps://mypy-play.net/?mypy=latest\u0026python=3.12\u0026gist=833674a36174c4140a89645b609da8a9","author":{"url":"https://github.com/randolf-scholz","@type":"Person","name":"randolf-scholz"},"datePublished":"2025-08-13T14:15:13.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/19652/mypy/issues/19652"}