https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113854
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> --- And those are not library internals, they're the public concepts defined in the standard. You can look them up to see what they mean, but the problem boils down to what's shown at the end: your predicate can't be used with that argument type. I think that's actually a more accurate description of the problem than telling you that a deleted copy constructor can't be used. We've just been conditioned by years of bad errors to know what it means. But really what you're trying to do is call a function, not copy a unique_ptr. The concept error tells you what part of the API is not satisfied. The second error tells you about a symptom, not the cause, and leaves to to infer the cause: it's trying to copy a move only object because the lambda requires a copy. There are plenty of cases where being told which concept failed is much clearer than being told about some unintended symptom feel inside a template. The existence of counterexamples doesn't mean concepts aren't an improvement.
