From 2d6d924d664db4e7c526a9a08ff5cc948e017bba Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Tue, 22 Sep 2020 07:53:58 +1000
Subject: [PATCH] types/tagged: support cvref in `visit`

---
 types/tagged.hpp | 63 +++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 52 insertions(+), 11 deletions(-)

diff --git a/types/tagged.hpp b/types/tagged.hpp
index f85a19f6..716003da 100644
--- a/types/tagged.hpp
+++ b/types/tagged.hpp
@@ -14,6 +14,8 @@
 #include "../maths.hpp"
 #include "../tuple/type.hpp"
 
+#include <tuple>
+
 
 ///////////////////////////////////////////////////////////////////////////////
 namespace cruft {
@@ -64,6 +66,7 @@ namespace cruft {
         tagged (tagged &&) = default;
 
         tagged& operator= (tagged const&) = default;
+        tagged& operator= (tagged &) = default;
         tagged& operator= (tagged &&) = default;
 
 
@@ -134,8 +137,30 @@ namespace cruft {
             cruft::max (sizeof (ValueT)...)
         ];
     };
+}
 
 
+namespace std {
+    template <typename ...ComponentT>
+    struct tuple_size<::cruft::tagged<ComponentT...>>
+        : std::integral_constant<::std::size_t, sizeof...(ComponentT)>
+    { ; };
+
+
+    template <std::size_t I, typename ...ComponentT>
+    struct tuple_element<
+        I,
+        ::cruft::tagged<ComponentT...>
+    >
+    : ::std::tuple_element<
+        I,
+        std::tuple<ComponentT...>
+    >
+    { ; };
+}
+
+
+namespace cruft {
     namespace detail {
         // If the tagged object matches the IndexV'th type in ComponentsT then
         // invoke visitor with the object as the argument. Else, advance to
@@ -146,25 +171,20 @@ namespace cruft {
         template <
             std::size_t IndexV,
             typename VisitorT,
-            template <typename...> typename TaggedT,
-            typename ...ComponentsT
-        >
-        requires std::is_same_v<
-            std::remove_cvref_t<TaggedT<ComponentsT...>>,
-            tagged<ComponentsT...>
+            typename TaggedT
         >
         decltype (auto)
         visit (
             VisitorT &&visitor,
-            TaggedT<ComponentsT...> arg
+            TaggedT &&arg
         ) {
-            static_assert (IndexV < sizeof...(ComponentsT));
-            using nth_t = std::tuple_element_t<IndexV, std::tuple<ComponentsT...>>;
+            static_assert (IndexV < std::tuple_size_v<std::remove_cvref_t<TaggedT>>);
+            using nth_t = std::tuple_element_t<IndexV, std::remove_reference_t<TaggedT>>;
 
             // If we're the last valid index then we must be invoked because
             // there's no other option. Do this, and (statically) avoid
             // further recursion.
-            if constexpr (IndexV + 1 == sizeof...(ComponentsT)) {
+            if constexpr (IndexV + 1 == std::tuple_size_v<std::remove_reference_t<TaggedT>>) {
                 return std::invoke (visitor, arg.template get<nth_t> ());
             } else {
                 // If the tag matches, then dispatch, else recurse with the
@@ -192,7 +212,28 @@ namespace cruft {
         tagged<ComponentsT...>
     >
     decltype (auto)
-    visit (VisitorT &&visitor, TaggedT<ComponentsT...> arg)
+    visit (VisitorT &&visitor, TaggedT<ComponentsT...> &arg)
+    {
+        static_assert (sizeof...(ComponentsT));
+
+        return detail::visit<0> (
+            std::forward<VisitorT> (visitor),
+            arg
+        );
+    }
+
+
+    template <
+        typename VisitorT,
+        template <typename...> typename TaggedT,
+        typename ...ComponentsT
+    >
+    requires std::is_same_v<
+        std::remove_cvref_t<TaggedT<ComponentsT...>>,
+        tagged<ComponentsT...>
+    >
+    decltype (auto)
+    visit (VisitorT &&visitor, TaggedT<ComponentsT...> const &arg)
     {
         static_assert (sizeof...(ComponentsT));