[4.2] Space Engine: uninhabited types map to empty spaces #17690
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Without this, the compiler ended up complaining about missing cases that can't actually occur, like `Optional<Never>.some(_)`. This was a regression from Swift 4.1. https://bugs.swift.org/browse/SR-8125 (cherry picked from commit e783027)
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@swift-ci Please test |
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@swift-ci Please nominate |
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Cool, now we can |
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Explanation: In adding
@unknown default, the switch statement exhaustivity checking started complaining about missing enum cases where the payload type was uninhabited (likeOptional<Never>.some(_)). It's clearly unnecessary for a user to have to match that case, and so this patch detects it earlier and avoids even adding thesomecase to the list of possible enum values. This may also very slightly speed up switch statement checking.Scope: Affects switch statement checking of enums when one of the case payload types is uninhabited.
Issue: SR-8125 / rdar://problem/41525746
Risk: Medium-low. This does affect switch exhaustivity checking, which means we could see more "case is redundant" diagnostics in addition to removing the unwanted "missing case" diagnostic. However, the change is correct from the perspective of the algorithm.
Testing: Added compiler regression tests, verified that the original project builds successfully.
Reviewed by: @CodaFi, @xedin