[TypeCheckPattern] Attempt ExprPattern conversion before failing pattern coercion to optional #67479
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…ern coercion to optional
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@swift-ci please test |
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@swift-ci please test |
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@swift-ci please test source compatibility |
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@swift-ci please test windows platform |
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@swift-ci please test windows platform |
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@swift-ci please test source compatibility release |
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@swift-ci please test windows platform |
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@swift-ci please test windows platform |
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Thanks @hamishknight! |
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See previous PR[1] and previous forum threads[2][3] regarding similar behavior and the intended semantics of expression pattern matching.
Expected behavior is for structural pattern matching and expression pattern matching to be transparent for the user. Currently, when matching an
EnumElementPattern(of the form.foo) against an expression of typeT, if we fail to find an enum elementT.foothen we will attempt to fall back to matching.fooas an expression, which can find static members.foo.However, if
Tis an optional typeU?, then if lookup ofOptional.foofails to find a matching enum element member, we will look into the wrapped type for an enum elementU.foo. If this fails, we (presently) do not take the fallback path and instead immediately fail with the error "enum case 'foo' not found in type 'U?'" even though lookup forUnresolvedMemberExprs would also look through optional types when attempting expression matching.This PR allows us to match against
U.fooexpressions by delaying the failure diagnostic in the optional case until after we've attempted the expression fallback. This will have the effect of allowing us to match the pattern.fooagainst values of typeU?whenUhas a static memberfoo: Self. It will also potentially change the diagnostic in failure cases from "enum case 'foo' not found in type 'U?'" to "type 'U?' has no member 'foo'," which is IMO more accurate since 'foo' is indeed permitted to be any (static) member, not just an enum case.[1]: #22486
[2]: https://forums.swift.org/t/matching-optionals-in-a-switch-statement/12905
[3]: https://forums.swift.org/t/allow-enum-case-matching-without/20544