https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63825
Bug ID: 63825
Summary: [DR 1558] Unused arguments in alias template
specializations
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
CC: jason at gcc dot gnu.org
http://open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1558
I managed to produce a testcase that still fails with GCC, confirming Walter's
claim that a workaround is needed to implement void_t in G++:
template<typename...> using void_t = void;
template<typename Functor, typename = void_t<>>
struct Maybe_get_result_type
{ };
template<typename Functor>
struct Maybe_get_result_type<Functor,
void_t<typename Functor::result_type>>
{ typedef typename Functor::result_type result_type; };
template<typename T>
struct wrapper
: Maybe_get_result_type<T>
{
T* t;
};
class X { };
wrapper<X> w;
void_t.cc: In instantiation of ‘struct Maybe_get_result_type<X, void>’:
void_t.cc:13:8: required from ‘struct wrapper<X>’
void_t.cc:21:12: required from here
void_t.cc:10:43: error: no type named ‘result_type’ in ‘class X’
{ typedef typename Functor::result_type result_type; };
^
EDG doesn't compile this either, but Clang does.
