@NamedVariant does not work on methods on a trait or default methods on an interface

The following code fails to compile:

import groovy.transform.NamedVariant

trait Foo {
  @NamedVariant
  def foo(String username = "sdf", String email = "ghj") {}
}

with this error:

Groovyc: Error during @NamedVariant processing. Class Foo$Trait$Helper already has a named-arg method of type [org.codehaus.groovy.ast.Parameter@15247533[name: namedArgs, type: java.util.Map, hasDefaultValue: false]]

It works fine as a class.

It fails as an interface with a default method.

Perhaps this is intended, but if so I failed to find any documentation specifying that @NamedVariant only works with classes.