Type not inferred from Iterator<Data<String>>

@groovy.transform.CompileStatic
@groovy.transform.Immutable( knownImmutableClasses = [T] )
class Data<T> {
  T value
}

@groovy.transform.CompileStatic
class StringDataIterator implements Iterator<Data<String>> {
  boolean hasNext() { true }
  void    remove()  {}

  Data<String> next() {
    new Data<String>( value: 'tim' )
  }
}

@groovy.transform.CompileStatic
class Runner {
  static main( args ) {
    Data<String> elem = new StringDataIterator().next()
    println elem.value.length()
  }
}

[Static type checking] - Cannot find matching method java.lang.Object#length(). Please check if the declared type is right and if the method exists.