And it won't work when switching the parameter order of byval, so you're correct. That's quite nasty.
Some people might ask why I chose to make the parameter of BlackBox::store an rvalue reference rather than an lvalue reference and force the client code to use std::move. While it is true that an lvalue reference would also have worked, if I did that, it would be impossible to write code like crate.store(std::make_unique<Gadget>(beeper, flasher));. I also happen to think that forcing the client to use std::move at the call sites is good rather than bad, because it communicates the intended effect to the reader.