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https://issues.apache.org/jira/browse/GROOVY-11508?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17892851#comment-17892851
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James Daugherty commented on GROOVY-11508:
------------------------------------------
This quote from GROOVY-8864 is why this ticket is so painful: "However, a trait
isn't a class but rather a mechanism for creating classes." The trait is
copying methods down to the implementing class and grails-data-mapping relied
on this copy behavior.
If we weren't using inheritance, we could traditionally implement this by
creating a trait per class:
{code:java}
trait GormEntityParent {
Parent myMethod() {
this
}
}
trait GormEntityChild {
Child myMethod() {
this
}
}
class Parent implements GormEntityParent {
}
class Child extends Parent implements GormEntityChild {
}
Parent p = new Parent()
System.out.println(p.myMethod())
Child c = new Child()
System.out.println(c.myMethod()){code}
This compiles in Groovy 4. If you view the trait as a mechanism for creating
classes, then Introducing a generic allows us to not have to create a trait per
implementation because of the copy strategy mentioned in the documents.
Also, since GROOVY-5106 breaks backwards compatibility with Groovy 3.x, it
would make sense for this to be a warning in Groovy 4 & a restriction in Groovy
5 instead if this change is really required.
> Multiple traits with related generic types cannot be used
> ---------------------------------------------------------
>
> Key: GROOVY-11508
> URL: https://issues.apache.org/jira/browse/GROOVY-11508
> Project: Groovy
> Issue Type: Bug
> Components: Compiler
> Affects Versions: 4.0.0
> Reporter: James Daugherty
> Assignee: Eric Milles
> Priority: Major
> Attachments: screenshot-1.png, screenshot-2.png, screenshot-3.png
>
>
> When updating Grails from Groovy 3.x to 4.x we discovered that GROOVY-5106
> prevents us from updating to Groovy 4 for the
> [grails-data-mapping|https://github.com/grails/grails-data-mapping] (GORM)
> project. Groovy-5106 does not take into account relationships between
> generic types and groovy does not support inheritance in generic types on
> traits so we have no workable solution for using Groovy 4.
>
> For example, the following is not possible in Groovy 4:
> {code:java}
> class Parent extends GormEntity<Parent> {
> }
> class Child extends GormEntity<Child> {
> }
> class GormEntity<? extends GormEntity> { // ? extends GormEntity is not
> allowed
> }{code}
>
> We have documented the impacts of this issue on the grails-data-mapping
> project here: [https://github.com/grails/grails-data-mapping/issues/1811]
> We have discovered that the original change could be reverted and continue to
> work with the latest Java & Groovy.
>
> For Grails Data Mapping (GORM), there is support for inheritance between a
> Parent & Child domain object. This is often implemented like this:
> {code:java}
> class Parent extends GormEntity<Parent> {
> }
> class Child extends GormEntity<Child> {
> }
> trait GormEntity<D> { // Simplified for this ticket
> static D get(Serializable id)
>
> static List<D> getAll()
> }
> {code}
> This allows someone to do the following in code:
> {code:java}
> Parent.get(1L) // Will find either a Child or Parent
> Child.get(1L) // Will find only child types{code}
>
> Since Groovy-5106 does not take into account inheritance, can this change be
> reverted or changed to a warning until inheritance is taken into account?
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