This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-11-11
[Resolved by issue 1395, which was moved to DR at the November, 2016 meeting.]
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.)
Notes from the October, 2012 meeting:
It is possible that 13.7.6.3 [temp.spec.partial.order] may resolve this problem.
CWG 2022-11-11
Resolved by issue 1395; see the special rule about trailing packs in 13.10.3.5 [temp.deduct.partial] paragraph 11.