C++ PATCH to suppress bogus -Wunused warning for parameter packs (PR c++/68965)

[Marek Polacek]

The problem in this PR is that we print bogus "unused parameter" warnings
for parameters of a parameter pack.  E.g. for
  auto count = [](auto&&... xs)
  {
    return sizeof...(xs);
  };
GCC issues
  warning: unused parameter 'xs#0'
  warning: unused parameter 'xs#1'
  warning: unused parameter 'xs#2'
  ...

This happens since <https://gcc.gnu.org/ml/gcc-patches/2015-11/msg02433.html>,
because now we aren't calling tsubst_copy that often.  But that means that
we're missing the mark_used call in tsubst_copy for those xs# parameters.
So to quash that -Wunused-parameter warning, I decided to set TREE_USED at the
place where we create those #xs parameters.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2016-01-13  Marek Polacek  <polacek@redhat.com>

	PR c++/68965
	* pt.c (tsubst_decl): Set TREE_USED.

	* g++.dg/cpp1y/parameter-pack-1.C: New test.

diff --git gcc/cp/pt.c gcc/cp/pt.c
index edec774..b9d4f59 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -11924,6 +11924,9 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 		/* Rename the parameter to include the index.  */
 		DECL_NAME (r)
 		  = make_ith_pack_parameter_name (DECL_NAME (r), i);
+
+		/* Set TREE_USED for the benefit of -Wunused.  */
+		TREE_USED (r) = true;
               }
             else if (!type)
               /* We're dealing with a normal parameter.  */
diff --git gcc/testsuite/g++.dg/cpp1y/parameter-pack-1.C gcc/testsuite/g++.dg/cpp1y/parameter-pack-1.C
index e69de29..8119b6e 100644
--- gcc/testsuite/g++.dg/cpp1y/parameter-pack-1.C
+++ gcc/testsuite/g++.dg/cpp1y/parameter-pack-1.C
@@ -0,0 +1,23 @@
+// PR c++/68965
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wall -Wextra" }
+
+auto count = [](auto&&... xs)
+{
+    return sizeof...(xs);
+};
+
+struct count_struct
+{
+    template<typename... Ts>
+    auto operator()(Ts&&... xs)
+    {
+        return sizeof...(xs);
+    }
+};
+
+int main()
+{
+    count(1,2,3,4,5,6,7);
+    count_struct{}(1,2,3,4,5,6,7);
+}

	Marek