groovy 'Alpha$TraitAdapter' implements trait 'Alpha' but does not extend self type class 'Thing'

[Daniel Garcia]

I'm doing some experiments with traits in scriptrunner and am running the following code in the console

import groovy.transform.SelfType

class Thing {
    String color

    Thing(String color) {
        this.color = color
    }

    String doSomething() {
        "something"
    }

    @Override
    String toString() {
        "${color} thing"
    }
}

//@SelfType(Thing)
trait Alpha {
    String alpha() {
        "alpha ${this}"
    }

    String alphaSomething() {
        "alpha ${this.doSomething()}"
    }
}

StringBuilder out = new StringBuilder()

out << "<pre class=\"dark\">\n"

Thing blueThing = new Thing("blue")
out << "${blueThing} (${blueThing.class})\n"                     // blue thing (class Thing)

def alphaThing = blueThing.withTraits Alpha
out << "${alphaThing} (${alphaThing.class})\n"                   // Thing18_groovyProxy@236366b2 (class Thing18_groovyProxy)
out << "${alphaThing.doSomething()}\n"                           // something
out << "${alphaThing.alpha()}, ${alphaThing.alphaSomething()}\n" // alpha Thing18_groovyProxy@236366b2, alpha something

out << "</pre>\n"

out.toString()

When I remove the comment on @SelfType(Thing) I get the following compilation error

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed: Alpha$TraitAdapterwrapper: -1: class 'Alpha$TraitAdapter' implements trait 'Alpha' but does not extend self type class 'Thing' @ line -1, column -1. 1 error