nixpkgs/pkgs/development/libraries/libunity/fix-vala.patch

--- a/protocol/protocol-icon.vala	2013-08-27 12:53:57 +0000
+++ b/protocol/protocol-icon.vala	2021-04-13 22:34:41 +0000
@@ -185,7 +185,7 @@
   }
 
   /* Added to GIcon interface in 2.37 */
-  private Variant serialize ()
+  private Variant? serialize ()
   {
     Variant? ret = null;
     return ret;