CI run 11698 still ongoing, but past afw which is the only package affected by the changes so I don't expect to see anything.

Looks good, thank you.

Actually, one comment – could you provide helpful error messages rather than just empty strings for when the assertion fails? For example, it's not trivially obvious to the reader why U should be const. (I realise that this goes beyond what Boost was previously providing.)

Merged after final CI build and adding of comments.