should std::common_type use std::decay?

Question

Given types A,B, I am concerned with the exact definition of std::common_type, disregarding the variadic case std::common_type<

Answer 1

should std::common_type use std::decay?

Yes, see Library Working Group Defect #2141.

Short version (long version, see link above):

• declval<A>() returns a A&&

• common_type is specified via declval, n3337:

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

• common_type<int, int>::type therefore yields int&&, which is unexpected

• proposed resolution is to add decay

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

• common_type<int, int>::type now yields int