This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115f. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-12-06
[Moved to DR at the November, 2014 meeting.]
The treatment of unused arguments in an alias template specialization is not specified by the current wording of 13.7.8 [temp.alias]. For example:
#include <iostream> template <class T, class...> using first_of = T; template <class T> first_of<void, typename T::type> f(int) { std::cout << "1\n"; } template <class T> void f(...) { std::cout << "2\n"; } struct X { typedef void type; }; int main() { f<X>(0); f<int>(0); }
Is the reference to first_of<void, T::type> with T being int equivalent to simply void, or is it a substitution failure?
(See also issues 1430, 1520, and 1554.)
Notes from the October, 2012 meeting:
The consensus of CWG was to treat this case as substitution failure.
Proposed resolution (February, 2014):
Add the following as a new paragraph before 13.7.8 [temp.alias] paragraph 3:
When a template-id refers to the specialization of an alias template, it is equivalent...
However, if the template-id is dependent, subsequent template argument substitution still applies to the template-id. [Example:
template<typename...> using void_t = void; template<typename T> void_t<typename T::foo> f(); f<int>(); // error, int does not have a nested type foo—end example]
The type-id in an alias template declaration shall not refer...