Concepts have been dropped from the new C++ standard

C++ concepts were designed to put constraints on templates. If you pass your class to the sorting algorithm, it had better be comparable using the less than operator, etc.

Basically a tool to write better libraries that give better compilation warnings upon failure. Incomprehensible compilation errors were the big failure of the STL, and this was going to fix that.

All the big names were pushing Concepts, including Stroustrup. Apparently the issue was that the GNU implementation was just too slow and they didn't expect compiler writers to do better soon.