Add is_swappable/is_nothrow_swappable traits

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@267079 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/containers/sequences/array/array.special/swap.pass.cpp

@@ -14,10 +14,28 @@
 #include <array>
 #include <cassert>
 
+#include "test_macros.h"
 // std::array is explicitly allowed to be initialized with A a = { init-list };.
 // Disable the missing braces warning for this reason.
 #include "disable_missing_braces_warning.h"
 
+struct NonSwappable {
+  NonSwappable() {}
+private:
+  NonSwappable(NonSwappable const&);
+  NonSwappable& operator=(NonSwappable const&);
+};
+
+template <class Tp>
+decltype(swap(std::declval<Tp>(), std::declval<Tp>()))
+can_swap_imp(int);
+
+template <class Tp>
+std::false_type can_swap_imp(...);
+
+template <class Tp>
+struct can_swap : std::is_same<decltype(can_swap_imp<Tp>(0)), void> {};
+
 int main()
 {
     {
@@ -44,4 +62,17 @@ int main()
         assert(c1.size() == 0);
         assert(c2.size() == 0);
     }
+    {
+        typedef NonSwappable T;
+        typedef std::array<T, 42> C1;
+        typedef std::array<T, 0> C0;
+        static_assert(!can_swap<C1&>::value, "");
+        static_assert(can_swap<C0&>::value, "");
+        C0 l = {};
+        C0 r = {};
+        swap(l, r);
+#if TEST_STD_VER >= 11
+        static_assert(noexcept(swap(l, r)), "");
+#endif
+    }
 }