[saurik]

You seem to be assuming that optimizations are only of value if they can be depended on happening consistently as a defined property of the language on every single platform for which there exists a compiler: in practice, compilers perform numerous optimizations (such as static branch prediction) that only work probabilistically even for the case of a specific version of a specific compiler on a specific operating system; I know of very few people (maybe you are one of them, however) who consider this to be a detriment. For a more obvious comparison: yes, I could sit around constructing object pools in my Java program to avoid heap allocations and their associated garbage collection penalty, but if rudimentary escape analysis manages to pick up a lot of the value, even if "fragile" or nondeterministic, I might not need to spend the time anymore to worry about that kind of detail in my software as the cost benefit analysis shows that the toolchain is now a more efficient usage of resources towards that issue (I could be spending my energy working on algorithm design, for example, instead of object pools). As long as the semantics of the language support the optimization, and as it doesn't seem to be difficult to implement, it would be useful to have the compiler perform this optimization for the developer, and one would expect such an optimization to either already be there or be "in the works".

[detrino]

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.

[a8da6b0c91d]

I guess considering a user defined move constructor could do something arbitrarily stupid you have to be consistent.

[detrino]

Also, any function can be overloaded on rvalue references.