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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Misleading warning for generic argument type on wrong return type","articleBody":"**Bug Report**\r\n\r\nIt appears Mypy propagates types into the type arguments of a function call if the types on the left side are know, slightly obscuring the actual error.\r\n\r\n\r\n\r\n\r\n\r\n**To Reproduce**\r\n\r\nRun `mypy a.py` with\r\n\r\n```py\r\nimport typing as t\r\n\r\nT = t.TypeVar('T')\r\nU = t.TypeVar('U')\r\n\r\ndef coalesce(a: T | None, b: U) -\u003e t.Union[T, U]:\r\n  if a is None:\r\n    return b\r\n  return a\r\n\r\ndef foo() -\u003e int:\r\n  return coalesce(42, None)\r\n```\r\n\r\n**Expected Behavior**\r\n\r\nLooking at this, I would expect Mypy to complain that the declared return value of `foo()` (being `int`) does not match what is actually returned (`int | None`). \r\n\r\n**Actual Behavior**\r\n\r\nIt appears that Mypy propagates the return type into the `coalesce()` type arguments instead and expects them both to be `int`, thus complaining about the argument type instead.\r\n\r\n```\r\na.py: note: In function \"foo\":\r\na.py:15: error: Argument 2 to \"coalesce\" has incompatible type \"None\"; expected \"int\"  [arg-type]\r\n      return coalesce(42, None)\r\n                          ^\r\nFound 1 error in 1 file (checked 1 source file)\r\n```\r\n\r\nI find this rather unintuitive. My suggestion would be to make Mypy complain about the type of the returned value instead of the argument type.\r\n\r\n**Your Environment**\r\n\r\n\u003c!-- Include as many relevant details about the environment you experienced the bug in --\u003e\r\n\r\n- Mypy version used: 0.931\r\n- Mypy command-line flags: /\r\n- Mypy configuration options from `pyproject.toml` (and other config files): \r\n\r\n    ```toml\r\n    [tool.mypy]\r\n    pretty = true\r\n    warn_redundant_casts = true\r\n    #warn_unused_ignores = true\r\n    warn_no_return = true\r\n    warn_unreachable = true\r\n    show_error_context = true\r\n    show_error_codes = true\r\n    ```\r\n\r\n- Python version used: 3.10.2\r\n- Operating system and version: WSL 2, Ubuntu 20\r\n","author":{"url":"https://github.com/NiklasRosenstein","@type":"Person","name":"NiklasRosenstein"},"datePublished":"2022-02-09T12:21:46.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/12156/mypy/issues/12156"}