Don't rewrite is/as cast operands to Self in prefer_self_in_static_references - #6765
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SimplyDanny merged 2 commits intoJun 8, 2026
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`prefer_self_in_static_references` rewrote the type operand of an `is` / `as?` / `as!` cast to `Self` inside class-like scopes. Because `Self` is the dynamic type, this silently changed runtime behavior for non-final classes: `x is A` (true for an `A` instance regardless of the caller) became `x is Self`, which is false when the method is invoked on a subclass instance. The rule already guards the analogous `X.self` case; `is`/`as` was an inconsistent gap. Skip the type operand of is/as casts in class-like scopes via `visit(TypeExprSyntax)`. Static member references (`A.f()`) and value-type scopes (`struct`/`enum`, where `Self` == the type) are unaffected. Two examples that pinned the old behavior become non-triggering: the `is A.Type` example from realm#6749 (the metatype is an is-operand, now left alone) and the pre-existing `class T { ... (input as! T).child ... }` example (its only violation was the `as! T` rewrite). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Pull request overview
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Fixes prefer_self_in_static_references to avoid rewriting cast type operands (e.g. x is A, x as? A) to Self in class-like scopes where that rewrite can change runtime semantics for non-final types.
Changes:
- Skip visiting
TypeExprSyntaxwhen it’s the operand ofis/ascasts in class-like scopes to prevent incorrect rewrites. - Add helper logic to detect cast operands in sequence expressions.
- Update rule examples and changelog to reflect the corrected behavior.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.
| File | Description |
|---|---|
| Source/SwiftLintBuiltInRules/Rules/Style/PreferSelfInStaticReferencesRuleExamples.swift | Moves cast-operand cases into non-triggering examples and removes corresponding corrections. |
| Source/SwiftLintBuiltInRules/Rules/Style/PreferSelfInStaticReferencesRule.swift | Adds cast-operand detection and skips rewriting type operands in class-like scopes. |
| CHANGELOG.md | Documents the bug fix and links the issue/author. |
- `isCastOperand` now does an O(1) parent check: `ExprListSyntax` is used
solely for a sequence expression's elements, where a bare type expression
only appears as an `is`/`as` operand, so no scan is needed.
- Mention plain `as` in the visitor comment (it is handled too).
- Drop the unreferenced `class B: A {}` from the non-triggering example.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Summary
prefer_self_in_static_referencesrewrote the type operand of anis/as?/as!cast toSelfinside class-like scopes. BecauseSelfis the dynamic type, this silently changes runtime behavior for non-final classes (the code still compiles):This is more dangerous than the existing composition/existential cases (which produced uncompilable code that fails fast) — this compiles and quietly breaks logic. The rule already guards the analogous
X.selfcase in class-like scopes for exactly this dynamic-vs-static reason;is/aswas an inconsistent gap.Fix
Skip the type operand of
is/as/as?/as!casts in class-like scopes, viavisit(TypeExprSyntax)(detecting the operand as the element following an unresolvedis/asoperator in the sequence expression). This mirrors the existingX.selfskip.Unaffected (still corrected, as before):
A.f()→Self.f()struct/enum,Self== the type, sox is S→x is Selfis safe and stays corrected[A]()and other non-cast type-as-value expressionsTests
x is A,x as? Ain a class extension).is A.Typeexample added in Don't rewrite composition/existential types toSelfin prefer_self_in_static_references #6749 and the pre-existingclass T { … (input as! T).child … }example both become non-triggering (their cast operands are now left alone), reflecting the corrected behavior.B().isA(A())staystrueafter--fix, and thatswiftlint lint --strictand the rule's generated tests pass.Fixes #6764
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