result_and_scratch_storage/future design issues:

[MikeDvorskiy]

Actually, there are really result_and_scratch_storage/future design issues:

• result_and_scratch_storage type combines live-time extension of a temporary buffer and provides returning some result values from a kernel.
• Moreover result_and_scratch_storage type has limitation - only one type for return values and temporary values.
• result_and_scratch_storage allows to set more than one value return, but just the same type and there is not effective API in future object to get the all values, just the first.

Originally posted by @MikeDvorskiy in #1942 (comment)

[danhoeflinger]

I agree that there are issues which need to be addressed here. However, if we want to expand its functionality to be suitable to more cases, we just need to be sure not to harm performance of the existing cases. I think that is possible but we need to be aware of it.