[libcxx] Reorganize tests since the application of P0602R4

Summary:
P0602R4 makes the special member functions of optional and variant
conditionally trivial based on the types in the optional/variant.
We already implemented that, but the tests were organized as if this
were a non-standard extension. This patch reorganizes the tests in a
way that makes more sense since this is not an extension anymore.

Reviewers: EricWF, mpark, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54772

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@350884 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/optional/optional.object/optional.object.assign/move.pass.cpp

@@ -12,11 +12,12 @@
 
 // optional<T>& operator=(optional<T>&& rhs)
 //     noexcept(is_nothrow_move_assignable<T>::value &&
-//              is_nothrow_move_constructible<T>::value);
+//              is_nothrow_move_constructible<T>::value); // constexpr in C++20
 
 #include <optional>
-#include <type_traits>
 #include <cassert>
+#include <type_traits>
+#include <utility>
 
 #include "test_macros.h"
 #include "archetypes.hpp"
@@ -51,6 +52,21 @@ struct Y {};
 bool X::throw_now = false;
 int X::alive = 0;
 
+
+template <class Tp>
+constexpr bool assign_empty(optional<Tp>&& lhs) {
+    optional<Tp> rhs;
+    lhs = std::move(rhs);
+    return !lhs.has_value() && !rhs.has_value();
+}
+
+template <class Tp>
+constexpr bool assign_value(optional<Tp>&& lhs) {
+    optional<Tp> rhs(101);
+    lhs = std::move(rhs);
+    return lhs.has_value() && rhs.has_value() && *lhs == Tp{101};
+}
+
 int main()
 {
     {
@@ -97,6 +113,24 @@ int main()
         assert(static_cast<bool>(opt) == static_cast<bool>(opt2));
         assert(*opt == *opt2);
     }
+    {
+        using O = optional<int>;
+#if TEST_STD_VER > 17
+        LIBCPP_STATIC_ASSERT(assign_empty(O{42}), "");
+        LIBCPP_STATIC_ASSERT(assign_value(O{42}), "");
+#endif
+        assert(assign_empty(O{42}));
+        assert(assign_value(O{42}));
+    }
+    {
+        using O = optional<TrivialTestTypes::TestType>;
+#if TEST_STD_VER > 17
+        LIBCPP_STATIC_ASSERT(assign_empty(O{42}), "");
+        LIBCPP_STATIC_ASSERT(assign_value(O{42}), "");
+#endif
+        assert(assign_empty(O{42}));
+        assert(assign_value(O{42}));
+    }
 #ifndef TEST_HAS_NO_EXCEPTIONS
     {
         static_assert(!std::is_nothrow_move_assignable<optional<X>>::value, "");