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How does typing.Never enable exhaustive match checking?

Q: How does typing.Never enable exhaustive match checking?

`Never` is the bottom type (no values); if a match is exhaustive, the wildcard's variable narrows to `Never` and `assert_never` type-checks, otherwise it still has a variant and fails. Variant remains Narrows to Never | | Type before assert_never | After exhaustive match | After non-exhaustive | |---|---|---| | Narrowed type | Union of remaining | `Never` (ok) | Still some variant (error) | | Checker result | -- | Passes | `Arg of type X not assignable to Never` | | Runtime | -- | No hit | Assertion fires | `Never` m

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Never as bottom type trick for pattern matching completeness.

typingnevermatch

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Never is the bottom type (no values); if a match is exhaustive, the wildcard's variable narrows to Never and assert_never type-checks, otherwise it still has a variant and fails.

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