Bring noexcept for <string> inline with other containers.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@132573 91177308-0d34-0410-b5e6-96231b3b80d8

test/strings/basic.string/string.nonmembers/string.special/swap_noexcept.pass.cpp

@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+// void swap(basic_string& c)
+//     noexcept(!allocator_type::propagate_on_container_swap::value ||
+//              __is_nothrow_swappable<allocator_type>::value);
+
+// This tests a conforming extension
+
+#include <string>
+#include <cassert>
+
+#include "../../test_allocator.h"
+
+template <class T>
+struct some_alloc
+{
+    typedef T value_type;
+    
+    some_alloc() {}
+    some_alloc(const some_alloc&);
+    void deallocate(void*, unsigned) {}
+
+    typedef std::true_type propagate_on_container_swap;
+};
+
+int main()
+{
+#if __has_feature(cxx_noexcept)
+    {
+        typedef std::string C;
+        C c1, c2;
+        static_assert(noexcept(swap(c1, c2)), "");
+    }
+    {
+        typedef std::basic_string<char, std::char_traits<char>, test_allocator<char>> C;
+        C c1, c2;
+        static_assert(noexcept(swap(c1, c2)), "");
+    }
+    {
+        typedef std::basic_string<char, std::char_traits<char>, some_alloc<char>> C;
+        C c1, c2;
+        static_assert(!noexcept(swap(c1, c2)), "");
+    }
+#endif
+}