http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53609
--- Comment #3 from Dodji Seketeli <dodji at gcc dot gnu.org> 2013-01-22 10:05:13 UTC --- Author: dodji Date: Tue Jan 22 10:05:05 2013 New Revision: 195367 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=195367 Log: PR c++/53609 - Wrong variadic template pack expansion in alias template Consider this example: 1 template<class...I> struct List {}; 2 template<int T> struct Z {static const int value = T;}; 3 template<int...T> using LZ = List<Z<T>...>; 4 5 template<class...U> 6 struct F 7 { 8 using N = LZ<U::value...>; //#1 This should amount to List<Z<U::value>...> 9 } 10 11 F<Z<1>, Z<2> >::N A; //#2 which G++ fails to compile, with this error message: test-PR53609-3.cc: In instantiation of 'struct F<Z<1>, Z<2> >': test-PR53609-3.cc:11:15: required from here test-PR53609-3.cc:3:43: error: wrong number of template arguments (2, should be 1) template<int...T> using LZ = List<Z<T>...>; ^ test-PR53609-3.cc:2:24: error: provided for 'template<int T> struct Z' template<int T> struct Z {static const int value = T;}; I think this is because in #1, when we substitute the argument pack {U::value...} into the pack expansion Z<T>..., tsubst_pack_expansion yields Z<U::value...>, instead of Z<U::value>..., so the instantiation of LZ amounts to List<Z<U::value...> >, instead of List<Z<U::value>...>. The idea of this patch is to make tsubst_pack_expansion support substituting an argument pack (into a pack expansion) where one of the arguments (let's call it the Ith argument) is itself a pack expansion P. In that case, the Ith element resulting from the substituting should be a pack expansion P'. The pattern of P' is then the pattern of P into which the pattern of the Ith argument of the argument pack has been substituted. Tested on x86_64-unknown-linux-gnu against trunk. gcc/cp/ * pt.c (argument_pack_element_is_expansion_p) (make_argument_pack_select, use_pack_expansion_extra_args_p) (gen_elem_of_pack_expansion_instantiation): New static functions. (tsubst): When looking through an ARGUMENT_PACK_SELECT tree node, look through the possibly resulting pack expansion as well. (tsubst_pack_expansion): Use use_pack_expansion_extra_p to generalize when to use the PACK_EXPANSION_EXTRA_ARGS mechanism. Use gen_elem_of_pack_expansion_instantiation to build the instantiation piece-wise. Don't use arg_from_parm_pack_p anymore, as gen_elem_of_pack_expansion_instantiation and the change in tsubst above generalize this particular case. (arg_from_parm_pack_p): Remove this for it's not used by tsubst_pack_expansion anymore. gcc/testsuite/ * g++.dg/cpp0x/variadic139.C: New test. * g++.dg/cpp0x/variadic140.C: Likewise. * g++.dg/cpp0x/variadic141.C: Likewise. Added: trunk/gcc/testsuite/g++.dg/cpp0x/variadic139.C trunk/gcc/testsuite/g++.dg/cpp0x/variadic140.C trunk/gcc/testsuite/g++.dg/cpp0x/variadic141.C Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/pt.c trunk/gcc/testsuite/ChangeLog
