"Concepts have been widely expected to produce better error messages than are currently produced when template instantiation fails. ... Unfortunately, it turns out not to be so simple and use of concepts sometimes results in worse error messages. Constraint failures frequently manifest as overload resolution failures resulting in a potentially long list of candidates, each with its own list of reasons for rejection. Identifying the candidate that was intended for a given use and then figuring out why the constraint failure occurred, can be a worse experience than navigating a template instantiation stack."

Perhaps someone indeed has been "widely expecting" a huge new source of complexity in C++'s already very complex static binding rules to help produce better error messages. I personally have never met that someone.