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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Incorrect \"redundant cast\" for linked TypeVar `T` and `type[T]` variables","articleBody":"\u003c!--\r\nIf you're not sure whether what you're experiencing is a mypy bug, please see the \"Question and Help\" form instead.\r\nPlease consider:\r\n\r\n- checking our common issues page: https://mypy.readthedocs.io/en/stable/common_issues.html\r\n- searching our issue tracker: https://github.com/python/mypy/issues to see if it's already been reported\r\n- asking on gitter chat: https://gitter.im/python/typing\r\n--\u003e\r\n\r\n**Bug Report**\r\n\r\n\u003c!--\r\nIf you're reporting a problem with a specific library function, the typeshed tracker is better suited for this report: https://github.com/python/typeshed/issues\r\n\r\nIf the project you encountered the issue in is open source, please provide a link to the project.\r\n--\u003e\r\n\r\nGiven a TypeVar `T` with multiple, concrete constraints, if you write a generic function that takes an argument of type `T`, another argument of type `type[T]`, and return a value of type `T`, mypy does not seem to have consistent behavior when it comes to handling the return type of individual expressions. Attempting to return a value without casting hits an `Incompatible return value type` error, while attempting to cast causes a `Redundant cast` error.\r\n\r\n**To Reproduce**\r\n\r\n```python\r\nfrom decimal import Decimal\r\nfrom typing import TypeVar, cast\r\n\r\nT = TypeVar(\"T\", Decimal, float)\r\n\r\n\r\ndef foo(x: T, x_type: type[T]) -\u003e T:\r\n    if x_type == float:\r\n        return cast(\"T\", 1.0)\r\n    if x_type == Decimal:\r\n        return cast(\"T\", Decimal(\"2.0\"))\r\n    raise ValueError(\"unreachable\")\r\n\r\n\r\ndef foo2(x: T, x_type: type[T]) -\u003e T:\r\n    if x_type == float:\r\n        return 1.0\r\n    if x_type == Decimal:\r\n        return Decimal(\"2.0\")\r\n    raise ValueError(\"unreachable\")\r\n```\r\n\r\nhttps://mypy-play.net/?mypy=1.9.0\u0026python=3.12\u0026flags=strict\u0026gist=4a0bc37034dcbcd5a18fabebeb5cbff5\r\n\r\n**Expected Behavior**\r\n\r\n\u003c!--\r\nHow did you expect mypy to behave? It’s fine if you’re not sure your understanding is correct.\r\nWrite down what you thought would happen. If you expected no errors, delete this section.\r\n--\u003e\r\nI expected either one of `foo()` or `foo2()` to be accepted by mypy. I don't necessarily expect mypy to be smart enough to infer the the linkage between narrowing the type of `x_type` and relating it to the type of `x`, but I at least expect to be able to explicitly cast the return expression without hitting a `Redundant cast` error.\r\n\r\n**Actual Behavior**\r\n\r\n\u003c!-- What went wrong? Paste mypy's output. --\u003e\r\n```\r\nmain.py:9: error: Redundant cast to \"float\"  [redundant-cast]\r\nmain.py:11: error: Redundant cast to \"Decimal\"  [redundant-cast]\r\nmain.py:17: error: Incompatible return value type (got \"float\", expected \"Decimal\")  [return-value]\r\nmain.py:19: error: Incompatible return value type (got \"Decimal\", expected \"float\")  [return-value]\r\nFound 4 errors in 1 file (checked 1 source file)\r\n```\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: 1.7.1 (but the mypy Playground I linked to above was 1.9.0)\r\n- Mypy command-line flags:\r\n- Mypy configuration options from `mypy.ini` (and other config files):\r\n  ```toml\r\n  warn_unused_ignores = true\r\n  warn_redundant_casts = true\r\n  warn_unused_configs = true\r\n  warn_unreachable = true\r\n  warn_return_any = true\r\n  strict = true\r\n  disallow_untyped_decorators = true\r\n  disallow_any_generics = false\r\n  implicit_reexport = false\r\n  show_error_codes = true\r\n  ```\r\n- Python version used: 3.11.6 (mypy Playground was 3.12)\r\n\r\n\u003c!-- You can freely edit this text, please remove all the lines you believe are unnecessary. --\u003e\r\n","author":{"url":"https://github.com/richardxia","@type":"Person","name":"richardxia"},"datePublished":"2024-03-18T06:04:21.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/17045/mypy/issues/17045"}