With the new core rules in regard to variadic pack expansions the library specification of the traits template common_type is now broken, the reason is that it is defined as a series of specializations of the primary template

    template <class ...T>
    struct common_type;

The broken one is this pair:

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

  template <class T, class U, class... V>
  struct common_type<T, U, V...> {
   typedef typename common_type<typename common_type<T, U>::type, V...>::type type;
  };

With the new rules both specializations would now be ambiguous for an instantiation like common_type<X, Y>.

(See also issue 1395.)