Debug mode for string. This commit also marks the first time libc++ debug-mode has found a bug (found one in regex). Had to play with extern templates a bit to get this to work since string is heavily used within libc++.dylib.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189114 91177308-0d34-0410-b5e6-96231b3b80d8

test/strings/basic.string/string.iterators/db_iterators_3.pass.cpp

@@ -0,0 +1,52 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// <string>
+
+// Subtract iterators from different containers with <.
+
+#if _LIBCPP_DEBUG2 >= 1
+
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+
+#include <string>
+#include <cassert>
+#include <iterator>
+#include <exception>
+#include <cstdlib>
+
+#include "../min_allocator.h"
+
+int main()
+{
+    {
+    typedef std::string S;
+    S s1;
+    S s2;
+    int i = s1.begin() - s2.begin();
+    assert(false);
+    }
+#if __cplusplus >= 201103L
+    {
+    typedef std::basic_string<char, std::char_traits<char>, min_allocator<char>> S;
+    S s1;
+    S s2;
+    int i = s1.begin() - s2.begin();
+    assert(false);
+    }
+#endif
+}
+
+#else
+
+int main()
+{
+}
+
+#endif