Allow is_swappable to SFINAE on deleted/ambiguous swap functions

llvm-svn: 275094

2 files changed, 30 insertions, 4 deletions

diff --git a/libcxx/include/type_traits b/libcxx/include/type_traits
index a8b0bd067d5..821a73c764d 100644
--- a/libcxx/include/type_traits
+++ b/libcxx/include/type_traits
@@ -4446,15 +4446,20 @@ iter_swap(_ForwardIterator1 __a, _ForwardIterator2 __b)
 namespace __detail
 {
 // ALL generic swap overloads MUST already have a declaration available at this point.
-using _VSTD::swap;
-__nat swap(__any, __any);
 
 template <class _Tp, class _Up = _Tp,
           bool _NotVoid = !is_void<_Tp>::value && !is_void<_Up>::value>
 struct __swappable_with
 {
-    typedef decltype(swap(_VSTD::declval<_Tp>(), _VSTD::declval<_Up>())) __swap1;
-    typedef decltype(swap(_VSTD::declval<_Up>(), _VSTD::declval<_Tp>())) __swap2;
+    template <class _LHS, class _RHS>
+    static decltype(swap(_VSTD::declval<_LHS>(), _VSTD::declval<_RHS>()))
+    __test_swap(int);
+    template <class, class>
+    static __nat __test_swap(long);
+
+    // Extra parens are needed for the C++03 definition of decltype.
+    typedef decltype((__test_swap<_Tp, _Up>(0))) __swap1;
+    typedef decltype((__test_swap<_Up, _Tp>(0))) __swap2;
 
     static const bool value = !is_same<__swap1, __nat>::value
                            && !is_same<__swap2, __nat>::value;
diff --git a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_swappable.pass.cpp b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_swappable.pass.cpp
index 9d01128379a..b17ca76fd58 100644
--- a/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_swappable.pass.cpp
+++ b/libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_swappable.pass.cpp
@@ -49,8 +49,21 @@ struct M {
 
 void swap(M&&, M&&) {}
 
+struct DeletedSwap {
+  friend void swap(DeletedSwap&, DeletedSwap&) = delete;
+};
+
 } // namespace MyNS
 
+namespace MyNS2 {
+
+struct AmbiguousSwap {};
+
+template <class T>
+void swap(T&, T&) {}
+
+} // end namespace MyNS2
+
 int main()
 {
     using namespace MyNS;
@@ -70,6 +83,14 @@ int main()
         static_assert(!std::is_swappable<int() &>::value, "");
     }
     {
+        // test that a deleted swap is correctly handled.
+        static_assert(!std::is_swappable<DeletedSwap>::value, "");
+    }
+    {
+        // test that a swap with ambiguous overloads is handled correctly.
+        static_assert(!std::is_swappable<MyNS2::AmbiguousSwap>::value, "");
+    }
+    {
         // test for presence of is_swappable_v
         static_assert(std::is_swappable_v<int>, "");
         static_assert(!std::is_swappable_v<M>, "");