Allow deque to handle incomplete types.

Allow deque and deque::iterator instantiation with incomplete element
type. This is an ABI breaking change, and it is only enabled if
LIBCXX_ABI_VERSION >= 2 or LIBCXX_ABI_UNSTABLE=ON.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@252350 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/containers/sequences/deque/incomplete.pass.cpp

@@ -0,0 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <deque>
+
+// deque()
+// deque::iterator()
+
+#define _LIBCPP_ABI_INCOMPLETE_TYPES_IN_DEQUE
+#include <deque>
+#include <cassert>
+
+struct A {
+  std::deque<A> d;
+  std::deque<A>::iterator it;
+  std::deque<A>::reverse_iterator it2;
+};
+
+int main()
+{
+  A a;
+  assert(a.d.size() == 0);
+  a.it = a.d.begin();
+  a.it2 = a.d.rend();
+}