[6.2][AST/Sema] SE-0487: Implement @nonexhaustive attribute and new enum exhaustivity checking rule
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This attribute controls whether cross-module access to the declaration needs `@unknown default:` because it's allowed to gain new cases even if the module is non-resilient. (cherry picked from commit a0ae93d)
For now the semantics provided by `@extensible` keyword on per-enum basis. We might return this as an upcoming feature in the future with a way to opt-out. (cherry picked from commit bf19481)
…tribute Guard against condfails when older compilers get a swift interface that uses `@extensible` attribute. The attribute itself doesn't have any effect in swift interfaces yet since all of the public enums are already resilient in that mode. (cherry picked from commit 6d89bca)
(cherry picked from commit 0ad7d8b)
…s for extensible enums Just like `@preconcurrency` for concurrency, this attribute is going to allow exhaustiveness error downgrades for enums that were retroactively marked as `@extensible`. (cherry picked from commit 498430a)
This is an accepted spelling for the attribute. This commit also renames the feature flag from `ExtensibleAttribute` to `NonexhaustiveAttribute` to match the spelling of the attribute. (cherry picked from commit fe1ae75)
…n` argument The spelling `@nonexhaustive(warn)` replaces `@preEnumExtensibility` attriubte. (cherry picked from commit 43eec8f)
(cherry picked from commit 0444d29)
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FranzBusch
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Aug 19, 2025
The proposal [has been accepted](https://forums.swift.org/t/accepted-se-0487-nonexhaustive-enums/81508) which makes the feature experimental no longer. (cherry picked from commit a7ecd30)
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bnbarham
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Aug 20, 2025
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Explanation:
This is an implementation of a second revision of SE-0487 proposal to
make non-frozen enums extensible for non-resilient cross-module access.
The new attribute (
@nonexhaustive) controls whether cross-module accessto the declaration needs
@unknown default:because it's allowed to gain newcases even if the module is non-resilient. A variant of the flag
@nonexhaustive(warn)could be used by API authors to push error about non-exhaustive use farther into the
future.
Resolves: rdar://154965992
Main Branch PRs: [AST/Sema] Add
@extensibleattribute onenumdeclarations #80503, [Frontend] AddExtensibleAttributeto guard use of@extensibleat… #81073, [AST/Sema] Add@preEnumExtensibilityattribute to downgrade warning… #81176, [AST/Sema] SE-0487: Adjust implementation based on the LSG feedback #82807Risk: Very Low. These changes are guarded by an experimental feature
NonexhaustiveAttributewhich is not enabled by default yet.Reviewed By: @FranzBusch
Testing: Added new test-cases.