It is a rather intrusive patch, hence my question whether I am allowed to merge this.
There is no change to the public interface. The patch provides nice compile-time error messages when users make mistakes in using the histogram instead of spilling out internals details as it does now, that's why I think it would be good to have this already in the upcoming release instead of the next after. The patch passes all the tests in the CI (and new tests developed for this feature). Line coverage remains at 100 % for Boost.Histogram, therefore I am confident that applying the patch is safe.