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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Mypy is still too eager about outer context","articleBody":"Consider this example:\r\n```python\r\nT = TypeVar('T')\r\nS = TypeVar('S')\r\n\r\ndef add(x: List[T], y: List[S]) -\u003e List[Union[T, S]]: ...\r\n\r\na: List[int]\r\nb: List[str]\r\n\r\ne: List[Union[int, str]] = add(a, b)\r\n```\r\nIt fails on the last line with the following (I skip invariance notes):\r\n```\r\nerror: Argument 1 to \"add\" has incompatible type \"List[int]\"; expected \"List[Union[int, str]]\"\r\nerror: Argument 2 to \"add\" has incompatible type \"List[str]\"; expected \"List[Union[int, str]]\"\r\n```\r\nIf I remove the annotation for `e`, everything works fine.\r\n\r\nWhen previously working on the external context issues, I found that I don't like how constrains for unions against unions are inferred (not that they are wrong, just counterintuitive). For example, `Union[T, S] \u003c: Union[str, int]` infers `T \u003c : Union[str, int] and S \u003c: Union[str, int]`, which is technically fine, but leads to errors like above.\r\n\r\nThis blocks https://github.com/python/mypy/pull/5873","author":{"url":"https://github.com/ilevkivskyi","@type":"Person","name":"ilevkivskyi"},"datePublished":"2018-11-08T15:58:21.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/5874/mypy/issues/5874"}