Support allocators in tuples of zero size.

	* include/std/tuple (tuple<>::tuple(),
	tuple<>::tuple(allocator_arg_t, const _Alloc&),
	tuple<>::tuple(allocator_arg_t, const _Alloc&, const tuple&)): New.
	* testsuite/20_util/tuple/cons/allocators.cc: Adjust.

From-SVN: r237143

libstdc++-v3/ChangeLog

+2016-06-06  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+	Support allocators in tuples of zero size.
+	* include/std/tuple (tuple<>::tuple(),
+	tuple<>::tuple(allocator_arg_t, const _Alloc&),
+	tuple<>::tuple(allocator_arg_t, const _Alloc&, const tuple&)): New.
+	* testsuite/20_util/tuple/cons/allocators.cc: Adjust.
+
 2016-06-06  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/71320

libstdc++-v3/include/std/tuple

@@ -876,6 +876,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       void swap(tuple&) noexcept { /* no-op */ }
+      // We need the default since we're going to define no-op
+      // allocator constructors.
+      tuple() = default;
+      // No-op allocator constructors.
+      template<typename _Alloc>
+	tuple(allocator_arg_t, const _Alloc&) { }
+      template<typename _Alloc>
+	tuple(allocator_arg_t, const _Alloc&, const tuple&) { }
     };
 
   /// Partial specialization, 2-element tuple.

libstdc++-v3/testsuite/20_util/tuple/cons/allocators.cc

@@ -162,8 +162,30 @@ void test01()
 
 }
 
+void test02()
+{
+  bool test __attribute__((unused)) = true;
+  using std::allocator_arg;
+  using std::tuple;
+  using std::make_tuple;
+
+  typedef tuple<> test_type;
+
+  MyAlloc a;
+
+  // default construction
+  test_type t1(allocator_arg, a);
+  // copy construction
+  test_type t2(allocator_arg, a, t1);
+  // move construction
+  test_type t3(allocator_arg, a, std::move(t1));
+  // make_tuple
+  test_type empty = make_tuple();
+}
+
 int main()
 {
   test01();
+  test02();
   return 0;
 }