Proposed resolution:

This wording is relative to N3485.

1. Change [meta.trans.other] p3 as indicated:

   template <class T>
   struct common_type<T> {
     typedef typename decay<T>::type type;
   };
   
   template <class T, class U>
   struct common_type<T, U> {
     typedef typename decay<decltype(true ? declval<T>() : declval<U>())>::type type;
   };