Proposed resolution:

In [locale.stdcvt] paragraph 2, Header codecvt synopsis:

template<class Elem, unsigned long Maxcode = 0x10ffff,
  codecvt_mode Mode = (codecvt_mode)0>
class codecvt_utf8
  : public codecvt<Elem, char, mbstate_t> {
  // unspecified
public:
  explicit codecvt_utf8(size_t refs = 0);
  ~codecvt_utf8();
  };

template<class Elem, unsigned long Maxcode = 0x10ffff,
  codecvt_mode Mode = (codecvt_mode)0>
class codecvt_utf16
  : public codecvt<Elem, char, mbstate_t> {
  // unspecified
public:
  explicit codecvt_utf16(size_t refs = 0);
  ~codecvt_utf16();
  };

template<class Elem, unsigned long Maxcode = 0x10ffff,
  codecvt_mode Mode = (codecvt_mode)0>
class codecvt_utf8_utf16
  : public codecvt<Elem, char, mbstate_t> {
  // unspecified
public:
  explicit codecvt_utf8_utf16(size_t refs = 0);
  ~codecvt_utf8_utf16();
  };