I'm getting following exception:

org.jenkinsci.plugins.scriptsecurity.sandbox.RejectedAccessException: Scripts not permitted to use method groovy.lang.GroovyObject invokeMethod java.lang.String java.lang.Object (Foo echo org.codehaus.groovy.runtime.GStringImpl)

when delegating instance of class to closure in shared library.

In shared library I have file vars/buildPlugin.groovy

def call(body) {
    def config = new Foo()
    body.resolveStrategy = Closure.DELEGATE_FIRST
    body.delegate = config
    body()

    echo "Name set by closure: ${config.name}"
}

class Foo {
    def name
}

and here is how it is using in Jenkinsfile (or pipeline script):

@Library('library.name') _

buildPlugin {
    name = 'name_from_Jenkinsfile'
    echo "Name from Jenkinsfile: ${name}"
}

I'm expecting to get following output:
Name from Jenkinsfile: name_from_Jenkinsfile
Name set by closure: name_from_Jenkinsfile
But instead getting the exception above.

Possible workarounds:
1) If uncheck Use Groovy Sandbox checkbox in pipeline job config - I'm getting expected output. But I don't want to disable sandbox or whitelist `method groovy.lang.GroovyObject invokeMethod java.lang.String java.lang.Object`

2) If change `def config = new Foo()` to `def config = [:]` in vars/buildPlugin.groovy I'm getting expected output. But I need to use class for defining additional methods in it.

3) If change `echo` to `this.echo` in Jenkinsfile - I'm getting expected output. But this will require to add `this` in all Jenkinsfiles for all objects not defined in delegated class instance.