Recommit r276548 - Make pair/tuples assignment operators SFINAE properly.

I think I've solved issues with is_assignable and references to incomplete
types. The updated patch adds tests for this case.



git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@276603 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/tuple/tuple.tuple/tuple.assign/copy.pass.cpp

@@ -19,6 +19,22 @@
 #include <string>
 #include <cassert>
 
+#include "test_macros.h"
+
+struct NonAssignable {
+  NonAssignable& operator=(NonAssignable const&) = delete;
+  NonAssignable& operator=(NonAssignable&&) = delete;
+};
+struct CopyAssignable {
+  CopyAssignable& operator=(CopyAssignable const&) = default;
+  CopyAssignable& operator=(CopyAssignable &&) = delete;
+};
+static_assert(std::is_copy_assignable<CopyAssignable>::value, "");
+struct MoveAssignable {
+  MoveAssignable& operator=(MoveAssignable const&) = delete;
+  MoveAssignable& operator=(MoveAssignable&&) = default;
+};
+
 int main()
 {
     {
@@ -51,4 +67,37 @@ int main()
         assert(std::get<1>(t) == 'a');
         assert(std::get<2>(t) == "some text");
     }
+    {
+        // test reference assignment.
+        using T = std::tuple<int&, int&&>;
+        int x = 42;
+        int y = 100;
+        int x2 = -1;
+        int y2 = 500;
+        T t(x, std::move(y));
+        T t2(x2, std::move(y2));
+        t = t2;
+        assert(std::get<0>(t) == x2);
+        assert(&std::get<0>(t) == &x);
+        assert(std::get<1>(t) == y2);
+        assert(&std::get<1>(t) == &y);
+    }
+    {
+        // test that the implicitly generated copy assignment operator
+        // is properly deleted
+        using T = std::tuple<std::unique_ptr<int>>;
+        static_assert(!std::is_copy_assignable<T>::value, "");
+    }
+    {
+        using T = std::tuple<int, NonAssignable>;
+        static_assert(!std::is_copy_assignable<T>::value, "");
+    }
+    {
+        using T = std::tuple<int, CopyAssignable>;
+        static_assert(std::is_copy_assignable<T>::value, "");
+    }
+    {
+        using T = std::tuple<int, MoveAssignable>;
+        static_assert(!std::is_copy_assignable<T>::value, "");
+    }
 }