Trait methods cannot be overriden by metaClass

[Søren Berg Glasius (GR8Conf EU)]

Hi,

We have run into an issue where methods in Traits cannot be be overriden with 
the metaClass

This is an example:

package demo

import spock.lang.Specification

class FooSpec extends Specification {

    void 'test replacing Widget.getMagicNumber'() {
        setup:
        Widget.metaClass.getMagicNumber = { ->
            2112
        }

        expect:
        // this one fails
        new Widget().getMagicNumber() == 2112
    }

    void 'test replacing Gadget.getMagicNumber'() {
        setup:
        Gadget.metaClass.getMagicNumber = { ->
            2112
        }

        expect:
        // this one passes
        new Gadget().getMagicNumber() == 2112
    }
}

class Widget implements Magician {}

trait Magician {
    int getMagicNumber() {
        42
    }
}

class Gadget {
    int getMagicNumber() {
        42
    }
}

This is causing a lot of headaches for Grails 3 projects, especially where 
metaClass overrides are used in tests. It has been reported as a bug here: 
https://issues.apache.org/jira/browse/GROOVY-7500 but has not yet been 
commented on by the Groovy team.

I hope this will raise awareness of the problem.

Best regards,
Søren Berg Glasius
GR8Conf Europe organizing team

GR8Conf ApS
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