René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[1.20 regression] Type narrowing is order sensitive in match-case statement with narrowed enum type","articleBody":"## Bug Report\n\nUnder specific circumstances, changing the order of `case`-statements in a `match`-statement influences mypy's type narrowing, leading to flaky `assert_never` errors.\n\nSpecifically, this happens you narrow enum values to specific enum values using `Literal` and combine that narrowed type with other types in a union.\n\n(I've not done an exhaustive search, but I've only seen this with `Literal[SomeEnum.SPECIFIC_VALUE]`.)\n\n### This bug does not occur with mypy==1.19\n\nI've tried to reproduce this bug with Mypy 1.19, but it only occurs with Mypy 1.20. (We ran into this after upgrading Mypy.)\n\n## To Reproduce\n\nIn the script below, the only different between the `dummy_class_then_enum` and `enum_then_dummy_class` is the order of the `case`-statements within the `match` statement.\n\nIn both instances, all possible value types are handled, but `mypy` will throw an error on the `assert_never` in the `enum_then_dummy_class` function.\n\n\n```python\nimport enum\nfrom typing import Literal, assert_never\n\n\nclass DummyClass:\n    \"\"\"A dummy class.\"\"\"\n\n\nclass MyEnum(enum.StrEnum):\n    \"\"\"A dummy enum class.\"\"\"\n\n    RELEVANT = enum.auto()\n    IGNORED = enum.auto()\n\n\ntype MyUnionType = DummyClass | Literal[MyEnum.RELEVANT]\n\n\ndef dummy_class_then_enum(arg: MyUnionType) -\u003e str:\n    match arg:\n        case DummyClass():\n            return \"dummy class\"\n        case MyEnum.RELEVANT:\n            return \"relevant\"\n        case _ as unreachable:\n            assert_never(unreachable)\n\n\ndef enum_then_dummy_class(arg: MyUnionType) -\u003e str:\n    match arg:\n        case MyEnum.RELEVANT:\n            return \"relevant\"\n        case DummyClass():\n            return \"dummy class\"\n        case _ as unreachable:\n            assert_never(unreachable)\n\n\n\n```\n\n### Expected Behavior\n\nMypy reports no errors (or errors for both functions).\n\n### Actual Behavior\n\nWe only get an error for the second function, not the first.\n\n```\n(mypy-demo) sebastiaan@python MypyDemo % mypy bug_demo.py\nbug_demo.py:36: error: Argument 1 to \"assert_never\" has incompatible type \"Literal[MyEnum.RELEVANT]\"; expected \"Never\"  [arg-type]\nFound 1 error in 1 file (checked 1 source file)\n```\n\n## Your Environment\n\n- Mypy version used: `1.20`\n- Mypy command-line flags: None\n- Mypy configuration options from `mypy.ini` (and other config files): None\n- Python version used: `3.14.3`\n","author":{"url":"https://github.com/SebastiaanZ","@type":"Person","name":"SebastiaanZ"},"datePublished":"2026-04-08T13:45:46.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/21187/mypy/issues/21187"}