Generics and static type checking: "Cannot return value of type W on method returning type W"

import groovy.transform.TypeChecked

class Wrapper {}

@TypeChecked
class Foo<W extends Wrapper> {
    W doIt (List<W> l) {
        l.iterator().next()//Static type check failure
    }
}

//new Foo().doIt([])//uncomment this line to run as a script on groovyConsole