From playing with the editor, it looks like it is sound. What I thought the quoted passage was saying was that consistency between a class and a trait used by

From playing with the editor, it looks like it is sound. What I thought the quoted passage was saying was that consistency between a class and a trait used by the class was not checked. That's clearly not the case, though. I don't understand what the performance point is, though, since I don't think the copy-and-paste style would produce different answers. Finally, the online editor doesn't seem to honor // strict -- it doesn't produce an error when some methods aren't annotated.