Herb Sutter in a video answers a question about the concepts proposal considered for C++11 and from his remarks it sounds like multiple groups offered prototype implementations but all of them left concerns about slow compile times. The comment surprised me because it suggests that, at least in some cases, the prototypes being developed are not just proofs of concept -- they're even expected to perform. All the work that must take has me curious.
For mature languages, especially C++, how are experimental language features developed? Is it much different from developing a compiler that implements a standard? Does a developer have a sense of if it will work and perform or even if it ever could? What are the most time consuming parts and are any parts surprisingly easier than one might expect?
The question is not what does the C++ standards committee do, but rather the part that comes before. When an experimental implementation for a proposal is being put together and there aren't any completely solidified rules, how is the sausage made?
I'm not a professional compiler developer nor do I expect answers with step by step accounts. I'd like a high-level idea of how this would be done or if there are any general patterns at all. I don't know what to expect from the answers but even if there are no rules to the process and the small number of people who do this just cowboy it and then, for stuff that worked out, write up the "official version" as a proposal, that answer would still be informative.