For the following code: template <typename... Args> int g(Args...); template <int N = 0> struct A { template <typename... Args> static auto f(Args... args) -> decltype(g(args...)); }; int main() { A<>::f(); return 0; } gcc gives the following errors: test.cpp: In function 'int main()': test.cpp:7:27: sorry, unimplemented: use of 'type_pack_expansion' in template test.cpp:12:12: error: no matching function for call to 'A<>::f()' test.cpp:12:12: note: candidate is: test.cpp:7:55: note: template<class ... Args> static decltype (g(A::f::args ...)) A::f(Args ...) [with Args = {Args ...}, int N = 0, decltype (g(A::f::args ...)) = int] Are there any plans to implement this?
Also, does someone know a workaround for this?
Found a workaround: #include <type_traits> template <typename... Args> int g(Args...); template <typename... Args> struct deduce { typedef decltype(g(std::declval<Args>()...)) type; }; template <int N = 0> struct A { template <typename... Args> static typename deduce<Args...>::type f(Args... args); }; int main() { A<>::f(); return 0; }
CC-ing Jason about this one too. By the way, isn't the first time this sorry message surfaces, eg, Comment #4 in PR44167.
*** Bug 44167 has been marked as a duplicate of this bug. ***
(In reply to comment #3) > CC-ing Jason about this one too. By the way, isn't the first time this sorry > message surfaces, eg, Comment #4 in PR44167. ... actually adding Jason to the CC list ;) I've marked the other as a dup after comparing the results for both testcases with G++ 4.4 and later versions. Jason, is this likely to be easy to fix? I've been hitting this and PR 47335 while implementing std::allocator_traits. If they aren't likely to be fixed in the near future I'll look into workarounds
Jason, I've found a decent workaround now so if you have higher priority issues you'd rather fix then don't spend time on this one for my benefit. Thanks for the incredible number of fixes you're making at the moment!
Author: jason Date: Wed May 25 14:32:06 2011 New Revision: 174201 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=174201 Log: PR c++/48292 * pt.c (tsubst_decl) [PARM_DECL]: Handle partial instantiation of function parameter pack. (tsubst_pack_expansion): Likewise. Added: trunk/gcc/testsuite/g++.dg/cpp0x/variadic109.C Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/pt.c trunk/gcc/testsuite/ChangeLog
Author: jason Date: Wed May 25 15:26:14 2011 New Revision: 174207 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=174207 Log: PR c++/48292 * pt.c (tsubst_decl) [PARM_DECL]: Handle partial instantiation of function parameter pack. (tsubst_pack_expansion): Likewise. Added: branches/gcc-4_6-branch/gcc/testsuite/g++.dg/cpp0x/variadic109.C Modified: branches/gcc-4_6-branch/gcc/cp/ChangeLog branches/gcc-4_6-branch/gcc/cp/pt.c branches/gcc-4_6-branch/gcc/testsuite/ChangeLog
Fixed for 4.6.1.