[ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones. #75135
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…mances in favor of explicit available ones. The type checker does not support the notion of multiple protocol conformances; there can only be one conformance, and if that conformance is unavailable, you cannot specify your own available conformance. This is important for Sendable checking; if a framework specifies that a type is explicitly not Sendable with an unavailable Sendable conformance, clients cannot ignore Sendable violations involving that type. If a superclass wants to allow subclasses to add a Sendable conformance, it should not declare an unavailable Sendable conformance.
…rom the defining module, and diagnose redundant Sendable conformances. We still allow re-stating inherited unchecked Sendable conformances in subclasses because inherited Sendable conformances are surprising when they opt out of static checking. Otherwise, warning on redundant Sendable conformances nudges programmers toward cleaning up unnecessary retroactive Sendable conformances over time as libraries incrementally add the conformances directly.
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hborla
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Jul 12, 2024
Sendable conformances for source compatibility. If conformance lookup always prefers the conformance from the defining module, libraries introducing unavailable Sendable conformances can break source in clients that declare retroactive Sendable conformances. Instead, still prefer the available conformance, and always diagnose the client conformance as redundant (as a warning). Then, when the retroactive conformance is removed, the errors will surface, so the programmer has to take explicit action to experience the source break.
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@swift-ci please smoke test |
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@swift-ci please test source compatibility |
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@swift-ci please smoke test |
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@swift-ci please test source compatibility |
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The macOS smoke test is failing due to a driver test failure unrelated to this change. Looks like a revert is being tested at swiftlang/swift-driver#1662 |
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@swift-ci please smoke test macOS |
This was referenced Feb 22, 2025
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The type checker does not support the notion of multiple protocol conformances; there can only be one conformance, and if that conformance is unavailable, you cannot specify your own available conformance. This is important for Sendable checking; if a framework specifies that a type is explicitly not Sendable with an unavailable Sendable conformance, clients cannot ignore Sendable violations involving that type. If a superclass wants to allow subclasses to add a Sendable conformance, it should not declare an unavailable Sendable conformance.
Note that this change still does not warn if the original unavailable conformance is declared with
@_nonSendabledue to the way@_nonSendableexpansion happens during implicitSendablederivation, after no explicit conformance has been found.Resolves: rdar://124423524