Implement N4606 optional

Summary:
Adapt implementation of Library Fundamentals TS optional into an implementation of N4606 optional.

  - Update relational operators per http://wg21.link/P0307
  - Update to requirements of http://wg21.link/P0032
  - Extension: Implement trivial copy/move construction/assignment for `optional<T>` when `T` is trivially copyable.

Audit P/Rs for optional LWG issues:
  - 2756 "C++ WP optional<T> should 'forward' T's implicit conversions" Implemented, which also resolves 2753 "Optional's constructors and assignments need constraints" (modulo my refusal to explicitly delete the move operations, which is a design error that I'm working on correcting in the 2756 P/R).
  - 2736 "nullopt_t insufficiently constrained" Already conforming. I've added a test ensuring that `nullopt_t` is not copy-initializable from an empty braced-init-list, which I believe is the root intent of the issue, to avoid regression.
  - 2740 "constexpr optional<T>::operator->" Already conforming.
  - 2746 "Inconsistency between requirements for emplace between optional and variant" No P/R, but note that the author's '"suggested resolution" is already implemented.
  - 2748 "swappable traits for optionals" Already conforming.
  - 2753 "Optional's constructors and assignments need constraints" Implemented.

Most of the work for this patch was done by Casey Carter @ Microsoft. Thank you Casey!



Reviewers: mclow.lists, CaseyCarter, EricWF

Differential Revision: https://reviews.llvm.org/D22741

llvm-svn: 283980

1 files changed, 33 insertions, 0 deletions

diff --git a/libcxx/test/std/utilities/optional/optional.object/optional.object.observe/value_const.fail.cpp b/libcxx/test/std/utilities/optional/optional.object/optional.object.observe/value_const.fail.cpp
new file mode 100644
index 00000000000..6076c509fa4
--- /dev/null
+++ b/libcxx/test/std/utilities/optional/optional.object/optional.object.observe/value_const.fail.cpp
@@ -0,0 +1,33 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+// constexpr const T& optional<T>::value() const &;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+using std::optional;
+
+struct X
+{
+    constexpr int test() const {return 3;}
+    int test() {return 4;}
+};
+
+int main()
+{
+    {
+        constexpr optional<X> opt;
+        static_assert(opt.value().test() == 3, "");
+    }
+}