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https://issues.apache.org/jira/browse/GROOVY-11985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18080137#comment-18080137
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Paul King commented on GROOVY-11985:
------------------------------------
*Static-method dispatch in traits (GROOVY-11985)*
Inside a trait body, an unqualified static-method call (e.g. \{{m()}}) or a
\{{this.}}-qualified static-method call (e.g. \{{this.m()}}) to a *public*
trait static is rewritten to invoke the method on the trait's direct
implementer — that is, the class whose \{{implements}} clause names the trait
and which therefore received the trait's compile-time forwarders. An
implementing class may override a trait's static defaults by declaring a
same-signature static method, and the override will be visible to trait code
calling that method.
The direct implementer is fixed at trait-composition time: subclasses of a
trait implementer do *not* re-compose the trait. Consequently, if class \{{A}}
implements trait \{{T}} (with static \{{m1}} calling static \{{m2}}) and class
\{{B}} extends \{{A}}, calling \{{B.m1()}} dispatches \{{m2}} through \{{A}} —
even if \{{B}} declares its own \{{static m2}}. To make \{{B}}'s override
visible to \{{m1}}, \{{B}} must also redeclare \{{m1}} (the new forwarder will
then carry \{{B}} as its receiver). This matches JVM semantics for inherited
static methods: the static receiver in source code does not influence dispatch.
Worked example:
{code:groovy}
trait T {
static String m2() \{ 'T.m2' }
static String m1() \{ m2() } // unqualified
static String m1This() \{ this.m2() } // this.-qualified
}
class A implements T \{ static String m2() { 'A.m2' } }
class B extends A \{ static String m2() { 'B.m2' } }
class C extends A {} // inherits A.m2
class D implements T {} // no override
class E extends D {} // inherits T.m2
class F extends D \{ static String m2() { 'F.m2' } }
{code}
||Receiver||\{{m2()}} direct||\{{m1()}} (unqualified)||\{{m1This()}}||
|\{{A}}|\{{A.m2}}|\{{A.m2}}|\{{A.m2}}|
|\{{B}}|\{{B.m2}}|\{{A.m2}}|\{{A.m2}}|
|\{{C}}|\{{A.m2}}|\{{A.m2}}|\{{A.m2}}|
|\{{D}}|\{{T.m2}}|\{{T.m2}}|\{{T.m2}}|
|\{{E}}|\{{T.m2}}|\{{T.m2}}|\{{T.m2}}|
|\{{F}}|\{{F.m2}}|\{{T.m2}}|\{{T.m2}}|
The bold cells for \{{B}} and \{{F}} show the load-bearing point: the subclass
override of \{{m2}} is *not* visible to \{{m1}} called through the subclass,
because \{{m1}}'s forwarder was baked into the direct implementer (\{{A}} or
\{{D}}) at composition time.
*Exceptions:*
* A *private* static is not composed onto the implementing class and continues
to dispatch to the trait's helper directly; the override pattern is
intentionally unavailable for private statics.
* A static call made from inside a *closure body* inside a trait method also
dispatches to the helper rather than through the implementer, preserving
compile-time type checking of trait closure bodies at the cost of polymorphism
in that position. This is a known difference from Groovy 4 and may be revisited
in a future revision.
*Qualified by trait name:*
A call qualified by the trait name (e.g. \{{MyTrait.m()}}) inside trait code
targets the trait's compiled body via the helper. Note that direct external
access of the form \{{MyTrait.m()}} is *not* supported because the trait
interface carries no static members.
*Overload selection:*
Overload selection uses the method's natural signature; the call dispatches
through the direct implementer's copy, hitting an override declared there if
one exists or the trait's compiled default otherwise.
> 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
>
> Analysis by AI of the description in the Grails canary build comments yielded
> the following:
> 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:java|title=Validateable.groovy}
> trait Validateable {
> static boolean defaultNullable() {
> false
> }
> static boolean defaultNullableSeenByTrait() {
> // expected to dispatch to the implementing class override
> this.defaultNullable()
> }
> }
> {code}
> {code:java|title=MyNullableValidateable.groovy}
> class MyNullableValidateable implements Validateable {
> static boolean defaultNullable() {
> true
> }
> }
> {code}
> {code:java|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):
> {quote}write {{T.m(p)}} as {{this.m($static$self,p)}} not {{$self.m(p)}}
> {quote}
> That commit rewrote {{TraitReceiverTransformer.transformMethodCallOnThis}} so
> that {{this.someStaticMethod()}} inside a trait body is rewritten as {{(this
> | T$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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