b0ffaa36f0
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.
From-SVN: r195367
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