
I have been trying to implement a concept for mpl ForwardSequence, and besides the lack of const expr requirements in the latest version of concepts I have stumbled across the problem of concept recursion. One example of such recursion is the ForwardIterator [1] concept. The requirement on the next<i>::type type is that it must be a ForwardIterator. One could try to write a concept like that: concept ForwardIteratorStatic<typename I> { typename deref; typename next; requires ForwardIteratorStatic<next>; } But for the above concept, the ConceptGCC compiler gives an error. Also, this should not compile according to the latest concepts proposal that states that a concept is not defined until the closing brace of the concept body. As for now, I think that one can try to apply technique described by Doug [2]: concept IteratorStatic<typename I> { } concept ForwardIteratorStatic<typename I> : IteratorStatic<I> { typename deref; typename next; requires IteratorStatic<next>; } But to get to my question finally, I wonder if the recursion in mpl is intentional and thought out, or is it an accident that should be re-examined. If the decision was indeed thought out, I really wonder what would be a reasonable semantics of concept recursion. Cheers, Marcin [1] http://www.boost.org/doc/libs/1_40_0/libs/mpl/doc/refmanual/forward-iterator... [2] http://www.osl.iu.edu/MailArchives/conceptgcc/2007/03/0084.php