From 1550529a39137532051c1df1124cc7ceb896f1e5 Mon Sep 17 00:00:00 2001
From: Danny Robson <danny@nerdcruft.net>
Date: Tue, 21 Jul 2020 15:05:21 +1000
Subject: [PATCH] types/tagged: add an equality operator

---
 types/tagged.hpp | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/types/tagged.hpp b/types/tagged.hpp
index 9bf7b9fb..f85a19f6 100644
--- a/types/tagged.hpp
+++ b/types/tagged.hpp
@@ -201,4 +201,24 @@ namespace cruft {
             arg
         );
     }
+
+
+    template <typename ...ComponentsT>
+    bool operator== (tagged<ComponentsT...> const &lhs, tagged<ComponentsT...> const &rhs)
+    {
+        // Check the tags actually match to start with. This lets us use
+        // short-circuiting in a second for the actual equality check.
+        if (lhs.tag () != rhs.tag ())
+            return false;
+
+        // Construct a fold-expression that tests every component type for
+        // equality. Use short-circuiting with `is` to protect against `get`
+        // queries for the wrong types.
+        return (
+            (
+                lhs.template is<ComponentsT> () &&
+                (lhs.template get<ComponentsT> () == rhs.template get<ComponentsT> ())
+            ) || ...
+        );
+    }
 }