Add <optional> header.

This patch is largely thanks to Casey Carter @ Microsoft. He did the initial
work of porting our experimental implementation and tests over to namespace
std.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@283977 91177308-0d34-0410-b5e6-96231b3b80d8

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

@@ -0,0 +1,102 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03, c++11, c++14
+// <optional>
+
+// optional<T>& operator=(const optional<T>& rhs);
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+#include "archetypes.hpp"
+
+using std::optional;
+
+struct X
+{
+    static bool throw_now;
+
+    X() = default;
+    X(const X&)
+    {
+        if (throw_now)
+            TEST_THROW(6);
+    }
+};
+
+bool X::throw_now = false;
+
+template <class Tp>
+constexpr bool assign_empty(optional<Tp>&& lhs) {
+    const optional<Tp> rhs;
+    lhs = rhs;
+    return !lhs.has_value() && !rhs.has_value();
+}
+
+template <class Tp>
+constexpr bool assign_value(optional<Tp>&& lhs) {
+    const optional<Tp> rhs(101);
+    lhs = rhs;
+    return lhs.has_value() && rhs.has_value() && *lhs == *rhs;
+}
+
+int main()
+{
+    {
+        using O = optional<int>;
+        LIBCPP_STATIC_ASSERT(assign_empty(O{42}), "");
+        LIBCPP_STATIC_ASSERT(assign_value(O{42}), "");
+        assert(assign_empty(O{42}));
+        assert(assign_value(O{42}));
+    }
+    {
+        using O = optional<TrivialTestTypes::TestType>;
+        LIBCPP_STATIC_ASSERT(assign_empty(O{42}), "");
+        LIBCPP_STATIC_ASSERT(assign_value(O{42}), "");
+        assert(assign_empty(O{42}));
+        assert(assign_value(O{42}));
+    }
+    {
+        using O = optional<TestTypes::TestType>;
+        assert(assign_empty(O{42}));
+        assert(assign_value(O{42}));
+    }
+    {
+        using T = TestTypes::TestType;
+        T::reset();
+        optional<T> opt(3);
+        const optional<T> opt2;
+        assert(T::alive == 1);
+        opt = opt2;
+        assert(T::alive == 0);
+        assert(!opt2.has_value());
+        assert(!opt.has_value());
+    }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        optional<X> opt;
+        optional<X> opt2(X{});
+        assert(static_cast<bool>(opt2) == true);
+        try
+        {
+            X::throw_now = true;
+            opt = opt2;
+            assert(false);
+        }
+        catch (int i)
+        {
+            assert(i == 6);
+            assert(static_cast<bool>(opt) == false);
+        }
+    }
+#endif
+}