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https://issues.apache.org/jira/browse/GROOVY-12602?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18135584#comment-18135584
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ASF GitHub Bot commented on GROOVY-12602:
-----------------------------------------
codeconsole opened a new pull request, #3129:
URL: https://github.com/apache/groovy/pull/3129
Under `@CompileStatic`, a lambda in a trait method that calls an instance
method of the trait by implicit `this` fails at run time:
```groovy
import java.util.function.Supplier
@groovy.transform.CompileStatic
trait U {
String getUname() { 'u' }
String a() { Supplier<String> s = () -> getUname(); s.get() }
}
class C implements U {}
assert new C().a() == 'u' // ClassCastException: class java.lang.Class
cannot be cast to class U
```
The same happens for a super trait's accessor (`uname` or `getUname()`), a
lambda in a lambda, a closure in a lambda and a lambda in a trait field
initializer, and in 4.0.33, 5.0.8 and 6.0.0. Dynamic Groovy, `@TypeChecked` and
closures work.
A trait method body becomes a static method of `T$Trait$Helper` that takes
the instance as `$self`. A closure there is rewritten to
`closure.rehydrate($self,$self,$self)`. Since GROOVY-11265 static compilation
drops that rewrite for a native lambda, and `StaticTypesLambdaWriter` then
passes the helper class as the lambda's owner and this object, because the
enclosing method is static. The implicit-this call reads `getThisObject()` and
casts it to the trait.
`StaticTypesLambdaWriter.loadLambdaReceiver` now passes the enclosing helper
method's `$self` as the owner and this object of a lambda that accesses
instance members, as `rehydrate` does for a closure. Closures nested in such a
lambda take its this object, so they work too. Static trait methods are
unchanged. Doing this in `TraitReceiverTransformer` instead would break a
lambda coerced to `Closure` that resolves names through a delegate (for example
`obj.with((b) -> prop)`): that transform runs before type checking and cannot
tell the two kinds of lambda apart.
## Tests
`TraitWithClosureOrLambda` gets five tests.
`testInvokeTraitMethodFromTraitLambda`,
`testInvokeSuperTraitMethodFromTraitLambda`,
`testInvokeTraitMethodFromTraitLambdaInFieldInitializer` and
`testInvokeTraitMethodFromTraitLambdaOrClosure` fail without the change.
`testInvokeTraitStaticOrPrivateMethodFromTraitLambda` covers neighbouring
shapes. `groovy.transform.stc.*`, `org.codehaus.groovy.classgen.asm.sc.*`,
`org.codehaus.groovy.transform.traitx.*` and `groovy.ThisAndSuperTest` pass.
The Checkstyle and CodeNarc gates pass.
> SC: a lambda in a trait method calling a trait instance method by implicit
> this throws ClassCastException: Class cannot be cast to the trait
> --------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: GROOVY-12602
> URL: https://issues.apache.org/jira/browse/GROOVY-12602
> Project: Groovy
> Issue Type: Bug
> Affects Versions: 4.0.33, 5.0.8, 6.0.0
> Reporter: Scott Murphy Heiberg
> Priority: Major
>
> {code:groovy}
> import java.util.function.Supplier
> @groovy.transform.CompileStatic
> trait U {
> String getUname() { 'u' }
> String a() { Supplier<String> s = () -> getUname(); s.get() }
> }
> class C implements U {}
> assert new C().a() == 'u'
> {code}
> Actual:
> {noformat}
> java.lang.ClassCastException: class java.lang.Class cannot be cast to class U
> at U$Trait$Helper$_a_lambda1.doCall(...)
> {noformat}
> The same happens for a super trait's accessor read as {{uname}} or called as
> {{getUname()}} in a lambda, for a lambda in a lambda, for a closure in a
> lambda and for a lambda in a trait field initializer.
> Expected: the assertion passes, as it does without {{@CompileStatic}}, with
> {{@TypeChecked}}, and with a closure instead of the lambda.
> A closure in a trait method is rehydrated with the trait instance as its this
> object. A statically compiled lambda is not, and the lambda writer passes the
> trait helper class as the lambda's this object because the helper method is
> static. The implicit-this call then casts that class to the trait.
> Seen in 4.0.33, 5.0.8 and 6.0.0.
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