Hello bearophile,
When a template constraint is composed of some different parts in &&, it's less easy to understand what condition has failed, so I miss the error messages written by me.
One solution to this issue would be to have an unconstrained temple that contains nothing but prgam(msg,"...") inside static ifs. That way failing the constraint produces a nice message and then, because the template has nothing it it, using it for almost anything will generate an error and block compilation.
-- ... <IXOYE><
