[boost] [type_traits] common_type and SFINAE

Hello all, common_type<T, U>::type is defined in a way that gives a hard compiler error if T and U do not have a common type (this is also how common_type is implemented in C++11). An alternative design (I'm not sure if implementable) would have been to not define the type member in case T and U have no common type. This way I could use SFINAE to detect if common_type<T, U>::type does not exist and therefore if T and U have not common type -- so to implement N3351's concept Common<T, U>. Is there a way to implement this alternative design? Thanks. --Lorenzo