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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@283980 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/optional/optional.object/optional.object.observe/bool.pass.cpp

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 explicit optional<T>::operator bool() const noexcept;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main()
+{
+    using std::optional;
+    {
+        const optional<int> opt; ((void)opt);
+        ASSERT_NOEXCEPT(bool(opt));
+        static_assert(!std::is_convertible<optional<int>, bool>::value, "");
+    }
+    {
+        constexpr optional<int> opt;
+        static_assert(!opt, "");
+    }
+    {
+        constexpr optional<int> opt(0);
+        static_assert(opt, "");
+    }
+}

test/std/utilities/optional/optional.object/optional.object.observe/dereference.pass.cpp

@@ -0,0 +1,73 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 T& optional<T>::operator*() &;
+
+#ifdef _LIBCPP_DEBUG
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+
+struct X
+{
+    constexpr int test() const& {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const&& {return 5;}
+    int test() && {return 6;}
+};
+
+struct Y
+{
+    constexpr int test() {return 7;}
+};
+
+constexpr int
+test()
+{
+    optional<Y> opt{Y{}};
+    return (*opt).test();
+}
+
+int main()
+{
+    {
+        optional<X> opt; ((void)opt);
+        ASSERT_SAME_TYPE(decltype(*opt), X&);
+        // ASSERT_NOT_NOEXCEPT(*opt);
+        // FIXME: This assertion fails with GCC because it can see that
+        // (A) operator*() is constexpr, and
+        // (B) there is no path through the function that throws.
+        // It's arguable if this is the correct behavior for the noexcept
+        // operator.
+        // Regardless this function should still be noexcept(false) because
+        // it has a narrow contract.
+    }
+    {
+        optional<X> opt(X{});
+        assert((*opt).test() == 4);
+    }
+    static_assert(test() == 7, "");
+#ifdef _LIBCPP_DEBUG
+    {
+        optional<X> opt;
+        assert((*opt).test() == 3);
+        assert(false);
+    }
+#endif  // _LIBCPP_DEBUG
+}

test/std/utilities/optional/optional.object/optional.object.observe/dereference_const.pass.cpp

@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>::operator*() const &;
+
+#ifdef _LIBCPP_DEBUG
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+
+struct X
+{
+    constexpr int test() const& {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const&& {return 5;}
+    int test() && {return 6;}
+};
+
+struct Y
+{
+    int test() const {return 2;}
+};
+
+int main()
+{
+    {
+        const optional<X> opt; ((void)opt);
+        ASSERT_SAME_TYPE(decltype(*opt), X const&);
+        // ASSERT_NOT_NOEXCEPT(*opt);
+        // FIXME: This assertion fails with GCC because it can see that
+        // (A) operator*() is constexpr, and
+        // (B) there is no path through the function that throws.
+        // It's arguable if this is the correct behavior for the noexcept
+        // operator.
+        // Regardless this function should still be noexcept(false) because
+        // it has a narrow contract.
+    }
+    {
+        constexpr optional<X> opt(X{});
+        static_assert((*opt).test() == 3, "");
+    }
+    {
+        constexpr optional<Y> opt(Y{});
+        assert((*opt).test() == 2);
+    }
+#ifdef _LIBCPP_DEBUG
+    {
+        const optional<X> opt;
+        assert((*opt).test() == 3);
+        assert(false);
+    }
+#endif  // _LIBCPP_DEBUG
+}

test/std/utilities/optional/optional.object/optional.object.observe/dereference_const_rvalue.pass.cpp

@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 T&& optional<T>::operator*() const &&;
+
+#ifdef _LIBCPP_DEBUG
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+
+struct X
+{
+    constexpr int test() const& {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const&& {return 5;}
+    int test() && {return 6;}
+};
+
+struct Y
+{
+    int test() const && {return 2;}
+};
+
+int main()
+{
+    {
+        const optional<X> opt; ((void)opt);
+        ASSERT_SAME_TYPE(decltype(*std::move(opt)), X const &&);
+        // ASSERT_NOT_NOEXCEPT(*std::move(opt));
+        // FIXME: This assertion fails with GCC because it can see that
+        // (A) operator*() is constexpr, and
+        // (B) there is no path through the function that throws.
+        // It's arguable if this is the correct behavior for the noexcept
+        // operator.
+        // Regardless this function should still be noexcept(false) because
+        // it has a narrow contract.
+    }
+    {
+        constexpr optional<X> opt(X{});
+        static_assert((*std::move(opt)).test() == 5, "");
+    }
+    {
+        constexpr optional<Y> opt(Y{});
+        assert((*std::move(opt)).test() == 2);
+    }
+#ifdef _LIBCPP_DEBUG
+    {
+        optional<X> opt;
+        assert((*std::move(opt)).test() == 5);
+        assert(false);
+    }
+#endif  // _LIBCPP_DEBUG
+}

test/std/utilities/optional/optional.object/optional.object.observe/dereference_rvalue.pass.cpp

@@ -0,0 +1,73 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 T&& optional<T>::operator*() &&;
+
+#ifdef _LIBCPP_DEBUG
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+
+struct X
+{
+    constexpr int test() const& {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const&& {return 5;}
+    int test() && {return 6;}
+};
+
+struct Y
+{
+    constexpr int test() && {return 7;}
+};
+
+constexpr int
+test()
+{
+    optional<Y> opt{Y{}};
+    return (*std::move(opt)).test();
+}
+
+int main()
+{
+    {
+        optional<X> opt; ((void)opt);
+        ASSERT_SAME_TYPE(decltype(*std::move(opt)), X&&);
+        // ASSERT_NOT_NOEXCEPT(*std::move(opt));
+        // FIXME: This assertion fails with GCC because it can see that
+        // (A) operator*() is constexpr, and
+        // (B) there is no path through the function that throws.
+        // It's arguable if this is the correct behavior for the noexcept
+        // operator.
+        // Regardless this function should still be noexcept(false) because
+        // it has a narrow contract.
+    }
+    {
+        optional<X> opt(X{});
+        assert((*std::move(opt)).test() == 6);
+    }
+    static_assert(test() == 7, "");
+#ifdef _LIBCPP_DEBUG
+    {
+        optional<X> opt;
+        assert((*std::move(opt)).test() == 3);
+        assert(false);
+    }
+#endif  // _LIBCPP_DEBUG
+}

test/std/utilities/optional/optional.object/optional.object.observe/has_value.pass.cpp

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 bool optional<T>::has_value() const noexcept;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main()
+{
+    using std::optional;
+    {
+        const optional<int> opt; ((void)opt);
+        ASSERT_NOEXCEPT(opt.has_value());
+        ASSERT_SAME_TYPE(decltype(opt.has_value()), bool);
+    }
+    {
+        constexpr optional<int> opt;
+        static_assert(!opt.has_value(), "");
+    }
+    {
+        constexpr optional<int> opt(0);
+        static_assert(opt.has_value(), "");
+    }
+}

test/std/utilities/optional/optional.object/optional.object.observe/op_arrow.pass.cpp

@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 T* optional<T>::operator->();
+
+#ifdef _LIBCPP_DEBUG
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+
+struct X
+{
+    int test() noexcept {return 3;}
+};
+
+struct Y
+{
+    constexpr int test() {return 3;}
+};
+
+constexpr int
+test()
+{
+    optional<Y> opt{Y{}};
+    return opt->test();
+}
+
+int main()
+{
+    {
+        std::optional<X> opt; ((void)opt);
+        ASSERT_SAME_TYPE(decltype(opt.operator->()), X*);
+        // ASSERT_NOT_NOEXCEPT(opt.operator->());
+        // FIXME: This assertion fails with GCC because it can see that
+        // (A) operator->() is constexpr, and
+        // (B) there is no path through the function that throws.
+        // It's arguable if this is the correct behavior for the noexcept
+        // operator.
+        // Regardless this function should still be noexcept(false) because
+        // it has a narrow contract.
+    }
+    {
+        optional<X> opt(X{});
+        assert(opt->test() == 3);
+    }
+    {
+        static_assert(test() == 3, "");
+    }
+#ifdef _LIBCPP_DEBUG
+    {
+        optional<X> opt;
+        assert(opt->test() == 3);
+        assert(false);
+    }
+#endif  // _LIBCPP_DEBUG
+}

test/std/utilities/optional/optional.object/optional.object.observe/op_arrow_const.pass.cpp

@@ -0,0 +1,76 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>::operator->() const;
+
+#ifdef _LIBCPP_DEBUG
+#define _LIBCPP_ASSERT(x, m) ((x) ? (void)0 : std::exit(0))
+#endif
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+
+struct X
+{
+    constexpr int test() const {return 3;}
+};
+
+struct Y
+{
+    int test() const noexcept {return 2;}
+};
+
+struct Z
+{
+    const Z* operator&() const;
+    constexpr int test() const {return 1;}
+};
+
+int main()
+{
+    {
+        const std::optional<X> opt; ((void)opt);
+        ASSERT_SAME_TYPE(decltype(opt.operator->()), X const*);
+        // ASSERT_NOT_NOEXCEPT(opt.operator->());
+        // FIXME: This assertion fails with GCC because it can see that
+        // (A) operator->() is constexpr, and
+        // (B) there is no path through the function that throws.
+        // It's arguable if this is the correct behavior for the noexcept
+        // operator.
+        // Regardless this function should still be noexcept(false) because
+        // it has a narrow contract.
+    }
+    {
+        constexpr optional<X> opt(X{});
+        static_assert(opt->test() == 3, "");
+    }
+    {
+        constexpr optional<Y> opt(Y{});
+        assert(opt->test() == 2);
+    }
+    {
+        constexpr optional<Z> opt(Z{});
+        static_assert(opt->test() == 1, "");
+    }
+#ifdef _LIBCPP_DEBUG
+    {
+        const optional<X> opt;
+        assert(opt->test() == 3);
+        assert(false);
+    }
+#endif  // _LIBCPP_DEBUG
+}

test/std/utilities/optional/optional.object/optional.object.observe/value.pass.cpp

@@ -0,0 +1,73 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 T& optional<T>::value() &;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+using std::bad_optional_access;
+
+struct X
+{
+    X() = default;
+    X(const X&) = delete;
+    constexpr int test() const & {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const && {return 5;}
+    int test() && {return 6;}
+};
+
+struct Y
+{
+    constexpr int test() & {return 7;}
+};
+
+constexpr int
+test()
+{
+    optional<Y> opt{Y{}};
+    return opt.value().test();
+}
+
+
+int main()
+{
+    {
+        optional<X> opt; ((void)opt);
+        ASSERT_NOT_NOEXCEPT(opt.value());
+        ASSERT_SAME_TYPE(decltype(opt.value()), X&);
+    }
+    {
+        optional<X> opt;
+        opt.emplace();
+        assert(opt.value().test() == 4);
+    }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        optional<X> opt;
+        try
+        {
+            opt.value();
+            assert(false);
+        }
+        catch (const bad_optional_access&)
+        {
+        }
+    }
+#endif
+    static_assert(test() == 7, "");
+}

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, "");
+    }
+}

test/std/utilities/optional/optional.object/optional.object.observe/value_const.pass.cpp

@@ -0,0 +1,64 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+#include "test_macros.h"
+
+using std::optional;
+using std::in_place_t;
+using std::in_place;
+using std::bad_optional_access;
+
+struct X
+{
+    X() = default;
+    X(const X&) = delete;
+    constexpr int test() const & {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const && {return 5;}
+    int test() && {return 6;}
+};
+
+int main()
+{
+    {
+        const optional<X> opt; ((void)opt);
+        ASSERT_NOT_NOEXCEPT(opt.value());
+        ASSERT_SAME_TYPE(decltype(opt.value()), X const&);
+    }
+    {
+        constexpr optional<X> opt(in_place);
+        static_assert(opt.value().test() == 3, "");
+    }
+    {
+        const optional<X> opt(in_place);
+        assert(opt.value().test() == 3);
+    }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        const optional<X> opt;
+        try
+        {
+            opt.value();
+            assert(false);
+        }
+        catch (const bad_optional_access&)
+        {
+        }
+    }
+#endif
+}

test/std/utilities/optional/optional.object/optional.object.observe/value_const_rvalue.pass.cpp

@@ -0,0 +1,64 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+#include "test_macros.h"
+
+using std::optional;
+using std::in_place_t;
+using std::in_place;
+using std::bad_optional_access;
+
+struct X
+{
+    X() = default;
+    X(const X&) = delete;
+    constexpr int test() const & {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const && {return 5;}
+    int test() && {return 6;}
+};
+
+int main()
+{
+    {
+        const optional<X> opt; ((void)opt);
+        ASSERT_NOT_NOEXCEPT(std::move(opt).value());
+        ASSERT_SAME_TYPE(decltype(std::move(opt).value()), X const&&);
+    }
+    {
+        constexpr optional<X> opt(in_place);
+        static_assert(std::move(opt).value().test() == 5, "");
+    }
+    {
+        const optional<X> opt(in_place);
+        assert(std::move(opt).value().test() == 5);
+    }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        const optional<X> opt;
+        try
+        {
+            std::move(opt).value();
+            assert(false);
+        }
+        catch (const bad_optional_access&)
+        {
+        }
+    }
+#endif
+}

test/std/utilities/optional/optional.object/optional.object.observe/value_or.pass.cpp

@@ -0,0 +1,68 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+// template <class U> T optional<T>::value_or(U&& v) &&;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+using std::in_place_t;
+using std::in_place;
+
+struct Y
+{
+    int i_;
+
+    Y(int i) : i_(i) {}
+};
+
+struct X
+{
+    int i_;
+
+    X(int i) : i_(i) {}
+    X(X&& x) : i_(x.i_) {x.i_ = 0;}
+    X(const Y& y) : i_(y.i_) {}
+    X(Y&& y) : i_(y.i_+1) {}
+    friend constexpr bool operator==(const X& x, const X& y)
+        {return x.i_ == y.i_;}
+};
+
+int main()
+{
+    {
+        optional<X> opt(in_place, 2);
+        Y y(3);
+        assert(std::move(opt).value_or(y) == 2);
+        assert(*opt == 0);
+    }
+    {
+        optional<X> opt(in_place, 2);
+        assert(std::move(opt).value_or(Y(3)) == 2);
+        assert(*opt == 0);
+    }
+    {
+        optional<X> opt;
+        Y y(3);
+        assert(std::move(opt).value_or(y) == 3);
+        assert(!opt);
+    }
+    {
+        optional<X> opt;
+        assert(std::move(opt).value_or(Y(3)) == 4);
+        assert(!opt);
+    }
+}

test/std/utilities/optional/optional.object/optional.object.observe/value_or_const.pass.cpp

@@ -0,0 +1,77 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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>
+
+// template <class U> constexpr T optional<T>::value_or(U&& v) const&;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+using std::optional;
+
+struct Y
+{
+    int i_;
+
+    constexpr Y(int i) : i_(i) {}
+};
+
+struct X
+{
+    int i_;
+
+    constexpr X(int i) : i_(i) {}
+    constexpr X(const Y& y) : i_(y.i_) {}
+    constexpr X(Y&& y) : i_(y.i_+1) {}
+    friend constexpr bool operator==(const X& x, const X& y)
+        {return x.i_ == y.i_;}
+};
+
+int main()
+{
+    {
+        constexpr optional<X> opt(2);
+        constexpr Y y(3);
+        static_assert(opt.value_or(y) == 2, "");
+    }
+    {
+        constexpr optional<X> opt(2);
+        static_assert(opt.value_or(Y(3)) == 2, "");
+    }
+    {
+        constexpr optional<X> opt;
+        constexpr Y y(3);
+        static_assert(opt.value_or(y) == 3, "");
+    }
+    {
+        constexpr optional<X> opt;
+        static_assert(opt.value_or(Y(3)) == 4, "");
+    }
+    {
+        const optional<X> opt(2);
+        const Y y(3);
+        assert(opt.value_or(y) == 2);
+    }
+    {
+        const optional<X> opt(2);
+        assert(opt.value_or(Y(3)) == 2);
+    }
+    {
+        const optional<X> opt;
+        const Y y(3);
+        assert(opt.value_or(y) == 3);
+    }
+    {
+        const optional<X> opt;
+        assert(opt.value_or(Y(3)) == 4);
+    }
+}

test/std/utilities/optional/optional.object/optional.object.observe/value_rvalue.pass.cpp

@@ -0,0 +1,72 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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 T& optional<T>::value() &&;
+
+#include <optional>
+#include <type_traits>
+#include <cassert>
+
+#include "test_macros.h"
+
+using std::optional;
+using std::bad_optional_access;
+
+struct X
+{
+    X() = default;
+    X(const X&) = delete;
+    constexpr int test() const & {return 3;}
+    int test() & {return 4;}
+    constexpr int test() const && {return 5;}
+    int test() && {return 6;}
+};
+
+struct Y
+{
+    constexpr int test() && {return 7;}
+};
+
+constexpr int
+test()
+{
+    optional<Y> opt{Y{}};
+    return std::move(opt).value().test();
+}
+
+int main()
+{
+    {
+        optional<X> opt; ((void)opt);
+        ASSERT_NOT_NOEXCEPT(std::move(opt).value());
+        ASSERT_SAME_TYPE(decltype(std::move(opt).value()), X&&);
+    }
+    {
+        optional<X> opt;
+        opt.emplace();
+        assert(std::move(opt).value().test() == 6);
+    }
+#ifndef TEST_HAS_NO_EXCEPTIONS
+    {
+        optional<X> opt;
+        try
+        {
+            std::move(opt).value();
+            assert(false);
+        }
+        catch (const bad_optional_access&)
+        {
+        }
+    }
+#endif
+    static_assert(test() == 7, "");
+}