Implement the first part of N4258: 'Cleaning up noexcept in the Library'. This patch deals with swapping containers, and implements a more strict noexcept specification (a conforming extension) than the standard mandates.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@242056 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -12,6 +12,10 @@
 // void swap(basic_string& c)
 //     noexcept(!allocator_type::propagate_on_container_swap::value ||
 //              __is_nothrow_swappable<allocator_type>::value);
+//
+//  In C++17, the standard says that swap shall have:
+//     noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
+//              allocator_traits<Allocator>::is_always_equal::value);
 
 // This tests a conforming extension
 
@@ -32,6 +36,19 @@ struct some_alloc
     typedef std::true_type propagate_on_container_swap;
 };
 
+template <class T>
+struct some_alloc2
+{
+    typedef T value_type;
+    
+    some_alloc2() {}
+    some_alloc2(const some_alloc2&);
+    void deallocate(void*, unsigned) {}
+
+    typedef std::false_type propagate_on_container_swap;
+    typedef std::true_type is_always_equal;
+};
+
 int main()
 {
 #if __has_feature(cxx_noexcept)
@@ -48,7 +65,21 @@ int main()
     {
         typedef std::basic_string<char, std::char_traits<char>, some_alloc<char>> C;
         C c1, c2;
+#if TEST_STD_VER >= 14
+    //  In c++14, if POCS is set, swapping the allocator is required not to throw
+        static_assert( noexcept(swap(c1, c2)), "");
+#else
         static_assert(!noexcept(swap(c1, c2)), "");
+#endif
+    }
+#if TEST_STD_VER >= 14
+    {
+        typedef std::basic_string<char, std::char_traits<char>, some_alloc2<char>> C;
+        C c1, c2;
+    //  if the allocators are always equal, then the swap can be noexcept
+        static_assert( noexcept(swap(c1, c2)), "");
     }
 #endif
+
+#endif
 }