[UncleMeat]

ABI. C++ defines rigorous calling conventions that don't permit this. There are some on the committee who want to permit ABI breaks to enable these sorts of optimizations (in this case, destructive-move) but the committee in general has been super hesitant to permit ABI breaks.

It won't break reasonable code. The problem is it breaks interop with compiled binaries.

[mehrdadn]

I'm confused, this seems unrelated to ABI. It's entirely up to the caller whether to subsequently destroy the moved-from object or to skip doing so. Whether or not the caller does so is irrelevant to the call or how the parameter gets passed, and it wouldn't affect the moved-to object (whose destructor always runs). The destruction of the moved-from object (if not elided) happens long after the call has returned - at the end of the object's scope.

I think you're confusing my proposal with the pass-in-register proposal? Or maybe I'm missing something.