types/tagged: add an equality operator

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> ())
+            ) || ...
+        );
+    }
 }