PR c++/53609 - Wrong variadic template pack expansion in alias template

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.

From-SVN: r195367