This is a very different kind of optimization than the other things you are talking about. This is something that is operating at a high level and potentially has implications that might not be obvious. You are changing the type of something and effecting the function overloads which are resolved. I think its important for what functions you are calling to be deterministic. My previous post was just touching on what possible value can be gained from allowing this and my conclusion is that it would be very little.

Also, your example of escape analysis is precisely because Java doesn't allow you to express what you are wanting to do, whereas C++ does allow you to express moves.

edit: I would compare this to C++'s copy elision, but that has a very high value and is much less intrusive.