Walter Bright:
Unique as a type qualifier comes with all kinds of subtle problems. For one thing, unique is more of a property of an expression rather than a property of the type.
Unfortunately I am not able to fully understand the paper linked by Sönke Ludwig in this thread, but maybe you are able to understand its main points.
Generally in this complex field I suggest to build on the ideas invented by researchers, and to not try to invent too much.
Bye, bearophile
