optional_sfinae.patch

diff --git a/include/optional b/include/optional
index 298f91a..9675ee7 100644
--- a/include/optional
+++ b/include/optional
@@ -241,7 +241,7 @@ struct __optional_destruct_base<_Tp, true>
            __engaged_(true) {}
 };
 
-template <class _Tp>
+template <class _Tp, bool = is_trivially_copyable<_Tp>::value>
 struct __optional_construct_base : __optional_destruct_base<_Tp>
 {
     using __base = __optional_destruct_base<_Tp>;
@@ -249,66 +249,16 @@ struct __optional_construct_base : __optional_destruct_base<_Tp>
 
     using __base::__base;
 
-    template <class... _Args>
-    _LIBCPP_INLINE_VISIBILITY
-    void __construct(_Args&&... __args)
-    {
-        _LIBCPP_ASSERT(!has_value(), "__construct called for engaged __optional_storage");
-        ::new(_VSTD::addressof(this->__val_)) value_type(_VSTD::forward<_Args>(__args)...);
-        this->__engaged_ = true;
-    }
+    constexpr __optional_construct_base()  noexcept : __base() {}
 
     _LIBCPP_INLINE_VISIBILITY
-    void reset() noexcept
+    __optional_construct_base(const __optional_construct_base& __opt)
     {
-        if (this->__engaged_)
-        {
-            this->__val_.~value_type();
-            this->__engaged_ = false;
-        }
+        if (__opt.__engaged_)
+            this->__construct(__opt.__val_);
     }
-};
-
-template <class _Tp, bool = is_move_constructible<_Tp>::value,
-    bool = is_trivially_copyable<__optional_destruct_base<_Tp>>::value>
-struct __optional_move_base;
-
-template <class _Tp, bool _IsTC>
-struct __optional_move_base<_Tp, false, _IsTC> : __optional_construct_base<_Tp>
-{
-    using __base = __optional_construct_base<_Tp>;
-    using __base::__base;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base() = default;
     _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base(__optional_move_base&&) = delete;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base(const __optional_move_base&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base& operator=(__optional_move_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base& operator=(const __optional_move_base&) = default;
-};
-
-template <class _Tp>
-struct __optional_move_base<_Tp, true, true> : __optional_construct_base<_Tp>
-{
-    using __base = __optional_construct_base<_Tp>;
-    using __base::__base;
-};
-
-template <class _Tp>
-struct __optional_move_base<_Tp, true, false> : __optional_construct_base<_Tp>
-{
-    using __base = __optional_construct_base<_Tp>;
-    using typename __base::value_type;
-    using __base::__base;
-
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base() = default;
-
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base(__optional_move_base&& __opt)
+    __optional_construct_base(__optional_construct_base&& __opt)
         noexcept(is_nothrow_move_constructible<value_type>::value)
     {
         if (__opt.__engaged_)
@@ -316,108 +266,25 @@ struct __optional_move_base<_Tp, true, false> : __optional_construct_base<_Tp>
     }
 
     _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base(const __optional_move_base&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base& operator=(__optional_move_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_base& operator=(const __optional_move_base&) = default;
-};
-
-template <class _Tp, bool = is_copy_constructible<_Tp>::value,
-    bool = is_trivially_copyable<__optional_destruct_base<_Tp>>::value>
-struct __optional_copy_base;
-
-template <class _Tp, bool _IsTC>
-struct __optional_copy_base<_Tp, false, _IsTC> : __optional_move_base<_Tp>
-{
-    using __base = __optional_move_base<_Tp>;
-    using __base::__base;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base() = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base(__optional_copy_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base(const __optional_copy_base&) = delete;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base& operator=(__optional_copy_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base& operator=(const __optional_copy_base&) = default;
-};
-
-template <class _Tp>
-struct __optional_copy_base<_Tp, true, true> : __optional_move_base<_Tp>
-{
-    using __base = __optional_move_base<_Tp>;
-    using __base::__base;
-};
-
-template <class _Tp>
-struct __optional_copy_base<_Tp, true, false> : __optional_move_base<_Tp>
-{
-    using __base = __optional_move_base<_Tp>;
-    using typename __base::value_type;
-    using __base::__base;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base() = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base(__optional_copy_base&&) = default;
-
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base(const __optional_copy_base& __opt)
+    __optional_construct_base& operator=(const __optional_construct_base& __opt)
     {
-        if (__opt.__engaged_)
-            this->__construct(__opt.__val_);
+        if (this->__engaged_ == __opt.__engaged_)
+        {
+            if (this->__engaged_)
+                this->__val_ = __opt.__val_;
+        }
+        else
+        {
+            if (this->__engaged_)
+                this->reset();
+            else
+                this->__construct(__opt.__val_);
+        }
+        return *this;
     }
 
     _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base& operator=(__optional_copy_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_base& operator=(const __optional_copy_base&) = default;
-};
-
-template <class _Tp, bool = is_move_assignable<_Tp>::value,
-    bool = is_trivially_copyable<__optional_destruct_base<_Tp>>::value>
-struct __optional_move_assign_base;
-
-template <class _Tp, bool _IsTC>
-struct __optional_move_assign_base<_Tp, false, _IsTC> : __optional_copy_base<_Tp>
-{
-    using __base = __optional_copy_base<_Tp>;
-    using __base::__base;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base() = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base(__optional_move_assign_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base(const __optional_move_assign_base&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base& operator=(__optional_move_assign_base&&) = delete;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base& operator=(const __optional_move_assign_base&) = default;
-};
-
-template <class _Tp>
-struct __optional_move_assign_base<_Tp, true, true> : __optional_copy_base<_Tp>
-{
-    using __base = __optional_copy_base<_Tp>;
-    using __base::__base;
-};
-
-template <class _Tp>
-struct __optional_move_assign_base<_Tp, true, false> : __optional_copy_base<_Tp>
-{
-    using __base = __optional_copy_base<_Tp>;
-    using typename __base::value_type;
-    using __base::__base;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base() = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base(__optional_move_assign_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base(const __optional_move_assign_base&) = default;
-
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base& operator=(__optional_move_assign_base&& __opt)
+    __optional_construct_base& operator=(__optional_construct_base&& __opt)
         noexcept(is_nothrow_move_assignable<value_type>::value &&
                  is_nothrow_move_constructible<value_type>::value)
     {
@@ -436,77 +303,59 @@ struct __optional_move_assign_base<_Tp, true, false> : __optional_copy_base<_Tp>
         return *this;
     }
 
+    template <class... _Args>
     _LIBCPP_INLINE_VISIBILITY
-    __optional_move_assign_base& operator=(const __optional_move_assign_base&) = default;
-};
-
-template <class _Tp, bool = is_copy_assignable<_Tp>::value,
-    bool = is_trivially_copyable<__optional_destruct_base<_Tp>>::value>
-struct __optional_copy_assign_base;
+    void __construct(_Args&&... __args)
+    {
+        _LIBCPP_ASSERT(!has_value(), "__construct called for engaged __optional_storage");
+        ::new((void*)_VSTD::addressof(this->__val_)) value_type(_VSTD::forward<_Args>(__args)...);
+        this->__engaged_ = true;
+    }
 
-template <class _Tp, bool _IsTC>
-struct __optional_copy_assign_base<_Tp, false, _IsTC> : __optional_move_assign_base<_Tp>
-{
-    using __base = __optional_move_assign_base<_Tp>;
-    using __base::__base;
     _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base() = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base(__optional_copy_assign_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base(const __optional_copy_assign_base&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base& operator=(__optional_copy_assign_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base& operator=(const __optional_copy_assign_base&) = delete;
-};
-
-template <class _Tp>
-struct __optional_copy_assign_base<_Tp, true, true> : __optional_move_assign_base<_Tp>
-{
-    using __base = __optional_move_assign_base<_Tp>;
-    using __base::__base;
+    void reset() noexcept
+    {
+        if (this->__engaged_)
+        {
+            this->__val_.~value_type();
+            this->__engaged_ = false;
+        }
+    }
 };
 
 template <class _Tp>
-struct __optional_copy_assign_base<_Tp, true, false> : __optional_move_assign_base<_Tp>
+struct __optional_construct_base<_Tp, true> : __optional_destruct_base<_Tp>
 {
-    using __base = __optional_move_assign_base<_Tp>;
+    using __base = __optional_destruct_base<_Tp>;
     using typename __base::value_type;
+
     using __base::__base;
+
+    template <class... _Args>
     _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base() = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base(__optional_copy_assign_base&&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base(const __optional_copy_assign_base&) = default;
-    _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base& operator=(__optional_copy_assign_base&&) = default;
+    void __construct(_Args&&... __args)
+    {
+        _LIBCPP_ASSERT(!has_value(), "__construct called for engaged __optional_storage");
+        ::new((void*)_VSTD::addressof(this->__val_)) value_type(_VSTD::forward<_Args>(__args)...);
+        this->__engaged_ = true;
+    }
 
     _LIBCPP_INLINE_VISIBILITY
-    __optional_copy_assign_base& operator=(const __optional_copy_assign_base& __opt)
+    void reset() noexcept
     {
-        if (this->__engaged_ == __opt.__engaged_)
-        {
-            if (this->__engaged_)
-                this->__val_ = __opt.__val_;
-        }
-        else
+        if (this->__engaged_)
         {
-            if (this->__engaged_)
-                this->reset();
-            else
-                this->__construct(__opt.__val_);
+            this->__val_.~value_type();
+            this->__engaged_ = false;
         }
-        return *this;
     }
 };
 
 template <class _Tp, class = void>
-class __optional_storage : __optional_copy_assign_base<_Tp>
+class __optional_storage : __optional_construct_base<_Tp>
 {
 public:
-    using __base = __optional_copy_assign_base<_Tp>;
+    using __base = __optional_construct_base<_Tp>;
     using typename __base::value_type;
 
     using __base::__base;
@@ -557,9 +406,75 @@ constexpr bool _Is_specialization = false;
 template <class... _Args, template <class...> class _Template>
 constexpr bool _Is_specialization<_Template<_Args...>, _Template> = true;
 
+
+template <bool _CanCopy, bool _CanMove>
+struct __optional_sfinae_ctor_base {};
+template <>
+struct __optional_sfinae_ctor_base<false, false> {
+  __optional_sfinae_ctor_base() = default;
+  __optional_sfinae_ctor_base(__optional_sfinae_ctor_base const&) = delete;
+  __optional_sfinae_ctor_base(__optional_sfinae_ctor_base &&) = delete;
+  __optional_sfinae_ctor_base& operator=(__optional_sfinae_ctor_base const&) = default;
+  __optional_sfinae_ctor_base& operator=(__optional_sfinae_ctor_base&&) = default;
+};
+template <>
+struct __optional_sfinae_ctor_base<true, false> {
+  __optional_sfinae_ctor_base() = default;
+  __optional_sfinae_ctor_base(__optional_sfinae_ctor_base const&) = default;
+  __optional_sfinae_ctor_base(__optional_sfinae_ctor_base &&) = delete;
+  __optional_sfinae_ctor_base& operator=(__optional_sfinae_ctor_base const&) = default;
+  __optional_sfinae_ctor_base& operator=(__optional_sfinae_ctor_base&&) = default;
+};
+template <>
+struct __optional_sfinae_ctor_base<false, true> {
+  __optional_sfinae_ctor_base() = default;
+  __optional_sfinae_ctor_base(__optional_sfinae_ctor_base const&) = delete;
+  __optional_sfinae_ctor_base(__optional_sfinae_ctor_base &&) = default;
+  __optional_sfinae_ctor_base& operator=(__optional_sfinae_ctor_base const&) = default;
+  __optional_sfinae_ctor_base& operator=(__optional_sfinae_ctor_base&&) = default;
+};
+template <class _Tp>
+using __optional_sfinae_ctor_base_t = __optional_sfinae_ctor_base<
+    is_copy_constructible<_Tp>::value,
+    is_move_constructible<_Tp>::value
+>;
+template <bool _CanCopy, bool _CanMove>
+struct __optional_sfinae_assign_base {};
+template <>
+struct __optional_sfinae_assign_base<false, false> {
+  __optional_sfinae_assign_base() = default;
+  __optional_sfinae_assign_base(__optional_sfinae_assign_base const&) = default;
+  __optional_sfinae_assign_base(__optional_sfinae_assign_base &&) = default;
+  __optional_sfinae_assign_base& operator=(__optional_sfinae_assign_base const&) = delete;
+  __optional_sfinae_assign_base& operator=(__optional_sfinae_assign_base&&) = delete;
+};
+template <>
+struct __optional_sfinae_assign_base<true, false> {
+  __optional_sfinae_assign_base() = default;
+  __optional_sfinae_assign_base(__optional_sfinae_assign_base const&) = default;
+  __optional_sfinae_assign_base(__optional_sfinae_assign_base &&) = default;
+  __optional_sfinae_assign_base& operator=(__optional_sfinae_assign_base const&) = default;
+  __optional_sfinae_assign_base& operator=(__optional_sfinae_assign_base&&) = delete;
+};
+template <>
+struct __optional_sfinae_assign_base<false, true> {
+  __optional_sfinae_assign_base() = default;
+  __optional_sfinae_assign_base(__optional_sfinae_assign_base const&) = default;
+  __optional_sfinae_assign_base(__optional_sfinae_assign_base &&) = default;
+  __optional_sfinae_assign_base& operator=(__optional_sfinae_assign_base const&) = delete;
+  __optional_sfinae_assign_base& operator=(__optional_sfinae_assign_base&&) = default;
+};
+template <class _Tp>
+using __optional_sfinae_assign_base_t = __optional_sfinae_assign_base<
+    is_copy_assignable<_Tp>::value,
+    is_move_assignable<_Tp>::value
+>;
+
 template <class _Tp>
 class optional
-    : private __optional_storage<_Tp>
+    : private __optional_storage<_Tp>,
+      private __optional_sfinae_ctor_base_t<_Tp>,
+      private __optional_sfinae_assign_base_t<_Tp>
 {
     template <class> friend class optional;
     using __base = __optional_storage<_Tp>;
@@ -570,7 +485,7 @@ public:
         "Instantiation of optional with a reference type is ill-formed.");
     static_assert(!is_same<remove_cv_t<value_type>, nullopt_t>::value,
         "Instantiation of optional with a nullopt_t type is ill-formed.");
-    static_assert(is_reference<value_type>::value || is_object<value_type>::value,
+    static_assert(is_object<value_type>::value,
         "Instantiation of optional with a non-object type is undefined behavior.");
 
     _LIBCPP_INLINE_VISIBILITY constexpr optional() noexcept {}