On Wed, Dec 15, 2010 at 2:19 PM, Dave Abrahams <dave@boostpro.com> wrote:
I don't see a problem. If you answer the question by giving foo an operator* of its own, the two implicitly-generated concept maps are the same concept map.
The problem is if the base concept specifies, for instance, an associated type requirement, possibly not even with a default. The concept map for "left" will have to specify this associated type and so will the concept map for "right". So, what happens when trying to access the concept map for base? Do you use the associated type specified by "left" or the one specified by "right"? This is especially problematic if the associated type is specified as something different in the concept map for "left" than from the concept map for "right". Marcin Zalewski answered this in a more recent reply in this thread -- apparently the last draft with concepts specifies that the two concept maps are checked for compatibility. There is an error if there is conflict. I should probably be able to do something similar in Generic, though it may end up being complicated (and complicated is a relative term with respect to the library already). With the implementation I'm imagining I can already see an ODR issue that would be tough, but not impossible, to account for.
I need more information. Please try to write out a complete example (using ConceptC++ syntax, please).
Marcin's recent reply should demonstrate what I meant. -- -Matt Calabrese