[Sema] Catch use-before-declarations in nested closures #85535
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hamishknight
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hamishknight
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Nov 17, 2025
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| // In principle we could allow these, but it's simpler to just reject them. | ||
| let x = z // expected-error {{use of local variable 'z' before its declaration}} | ||
| let y = { z } // expected-error {{use of local variable 'z' before its declaration}} | ||
| var z: Int { 0 } // expected-note 2{{'z' declared here}} |
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x here was already illegal, this change also makes y illegal. In principle we could allow forward references for computed vars, but I think it's much better to have a consistent behavior for all variables.
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I think that makes sense as well!
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jamieQ
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Previously we would allow these in Sema and diagnose them in SILGen, but allowing them in Sema is unsound because it means the constraint system ends up kicking interface type requests for declarations that should be type-checked as part of the closure itself. Adjust the name lookup logic to look through parent closures when detecting invalid forward references. For now we don't fallback to an outer result on encountering a use-before-declaration to preserve the current behavior. I'm planning on changing that in the next commit though. rdar://74430478
If we encounter a variable declared after its use within a closure, we can fallback to using an outer result if present. This matches the behavior outside of a closure and generally seems more consistent with the behavior we have if we also find an inner result. rdar://163656720
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@swift-ci please test macOS |
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@swift-ci please test Linux |
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@swift-ci please test Windows |
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@swift-ci please test source compatibility |
xedin
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Nov 17, 2025
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| // In principle we could allow these, but it's simpler to just reject them. | ||
| let x = z // expected-error {{use of local variable 'z' before its declaration}} | ||
| let y = { z } // expected-error {{use of local variable 'z' before its declaration}} | ||
| var z: Int { 0 } // expected-note 2{{'z' declared here}} |
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I think that makes sense as well!
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Previously we would allow these in Sema and diagnose them in SILGen, but allowing them in Sema is unsound because it means the constraint system ends up kicking interface type requests for declarations that should be type-checked as part of the closure itself. Adjust the name lookup logic to look through parent closures when detecting invalid forward references. This then allows us to have consistent shadowing behavior inside and outside of closures.
Forum post: https://forums.swift.org/t/ensuring-forward-references-to-local-variables-are-consistently-rejected/83265
rdar://74430478
rdar://163656720