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https://issues.apache.org/jira/browse/GROOVY-11985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18077891#comment-18077891
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Paul King edited comment on GROOVY-11985 at 5/3/26 11:15 AM:
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[~blackdrag] I tend to agree with you but traits are different to anything Java
offers, so I don't think we just want what is closest to Java.
FYI, the wording in the Jira is from AI, not necessarily what I think. I take
such content from AI as a starting point for a conversation. I guess we need to
be better at making AI suggestions explicitly marked. At the time, IDEA was
starting to approach what looked like OOM territory, so I was wanting to
capture the current state of play in the investigation in case IDEA crashed and
the conversation was lost (or partially lost).
was (Author: paulk):
[~blackdrag] I tend to agree with you. The wording in the Jira is from AI, not
necessarily what I think. I take such content from AI as a starting point for a
conversation. I guess we need to be better at making AI suggestions explicitly
marked. At the time, IDEA was starting to approach what looked like OOM
territory, so I was wanting to capture the current state of play in the
investigation in case IDEA crashed and the conversation was lost (or partially
lost).
> Static method override on trait implementer ignored when called via this in
> trait body
> --------------------------------------------------------------------------------------
>
> Key: GROOVY-11985
> URL: https://issues.apache.org/jira/browse/GROOVY-11985
> Project: Groovy
> Issue Type: Bug
> Reporter: Paul King
> Priority: Major
>
> h2. Summary
> In Groovy 4.x, calling {{this.someStaticMethod()}} from inside a trait's
> static method body dispatched dynamically and honoured a static method
> override declared on the implementing class. In Groovy 5.x and
> 6.0.0-SNAPSHOT, the same call is rewritten by {{TraitReceiverTransformer}} to
> dispatch through the trait helper, which can never see the implementing-class
> override. The override is silently lost — no exception, no compile warning,
> the trait's default value just always wins.
> This was discovered as part of the Grails 8 / Groovy 5 migration
> (apache/grails-core PRs
> [#15557|https://github.com/apache/grails-core/pull/15557] and
> [#15558|https://github.com/apache/grails-core/pull/15558]) and motivated a
> reflection-based workaround in {{Validateable.resolveDefaultNullable(Class)}}.
> h2. Reproducer
> Standalone repro:
> [https://github.com/jamesfredley/groovy-trait-static-method-override-bug]
> {code:title=Validateable.groovy}
> trait Validateable {
> static boolean defaultNullable() {
> false
> }
> static boolean defaultNullableSeenByTrait() {
> // expected to dispatch to the implementing class override
> this.defaultNullable()
> }
> }
> {code}
> {code:title=MyNullableValidateable.groovy}
> class MyNullableValidateable implements Validateable {
> static boolean defaultNullable() {
> true
> }
> }
> {code}
> {code:title=Driver}
> assert MyNullableValidateable.defaultNullable() // direct call:
> true on every version
> assert MyNullableValidateable.defaultNullableSeenByTrait() // expected true;
> gets false on 5.x / 6.0
> {code}
> h2. Observed behaviour
> || Groovy version || Direct call || {{this.defaultNullable()}} from trait
> body ||
> | 4.0.27 | true (PASS) | true (PASS)
> |
> | 5.0.5 | true (PASS) | *false (FAIL)*
> |
> | 6.0.0-SNAPSHOT | true (PASS) | *false (FAIL)*
> |
> h2. Root cause
> The bytecode emitted for the trait helper's
> {{defaultNullableSeenByTrait(Class)}} method changed shape:
> {noformat}
> // Groovy 4.0.27 // Groovy 5.0.5 /
> 6.0.0-SNAPSHOT
> 0: aload_0 0: ldc //
> Validateable$Trait$Helper.class
> 1: invokedynamic invoke: 2: aload_0
> (Ljava/lang/Class;)Ljava/lang/Object; 3: invokedynamic invoke:
>
> (Ljava/lang/Class;Ljava/lang/Class;)Ljava/lang/Object;
> {noformat}
> In 4.x the indy receiver is {{aload_0}} — the implementing class — so the
> dynamic dispatch resolves {{defaultNullable()}} against
> {{MyNullableValidateable}} and finds the override. In 5.x+ the receiver is
> hard-coded {{Validateable$Trait$Helper.class}} (an {{ldc}}) and the
> implementing class is demoted to an argument, so the dispatch resolves
> {{defaultNullable(Class)}} on the trait helper itself and always lands on the
> lowered trait default.
> The behaviour change was introduced by commit {{0aa78d0a33}} (GROOVY-8854,
> Sep 2023):
> bq. write {{T.m(p)}} as {{this.m($static$self,p)}} not {{$self.m(p)}}
> That commit rewrote {{TraitReceiverTransformer.transformMethodCallOnThis}} so
> that {{this.someStaticMethod()}} inside a trait body is rewritten as {{(this
> | T$Trait$Helper).m((Class)$self.getClass(), args)}} — routed through the
> trait helper's lowered static, with the implementing class passed as the
> {{$static$self}} argument. The existing trait static method test coverage in
> {{TraitASTTransformationTest.testTraitStaticMethod}} (including the
> GROOVY-8854 case at line 2218) doesn't exercise the
> override-on-implementing-class scenario, so the regression wasn't caught.
> h2. Tradeoff
> Groovy 5's behaviour is arguably closer to Java semantics: static methods are
> not virtual, and {{this}} inside a Java static method doesn't exist, so
> "{{this.staticMethod()}} virtually dispatches to the implementing class's
> static" was always Groovy-specific magic that relied on MOP. However:
> * The Groovy 4 behaviour was depended on by real code — Grails
> {{Validateable}} is the visible canary; the same idiom likely exists
> elsewhere.
> * The failure mode is silent — no exception, no compile warning, the trait
> default just wins every time.
> * The change surfaced as a side effect of GROOVY-8854 (whose ticket was about
> something else), not as a deliberate "trait statics are no longer virtual"
> decision, and it isn't called out in the Groovy 5 release notes.
> h2. Suggested options
> # Restore Groovy 4 semantics for {{this.staticMethod()}} in trait bodies —
> emit a dynamic lookup whose receiver is {{$static$self}} (the implementing
> class) rather than the trait helper.
> # At minimum, emit a compile-time warning when a trait body calls a
> same-named static and the dispatch will provably not hit any override on the
> implementing class.
> # Document the new contract explicitly in the Groovy 5 release notes and
> trait docs, so consumers can adapt at the trait level instead of debugging
> silent no-ops.
> h2. Workaround (already applied in Grails)
> Bypass {{TraitReceiverTransformer}} via plain Java reflection, which it can't
> see through:
> {code:groovy}
> private static boolean resolveDefaultNullable(Class<?> clazz) {
> try {
> return clazz.getMethod('defaultNullable').invoke(null) as boolean
> } catch (NoSuchMethodException ignored) {
> return false
> }
> }
> {code}
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