diff -r 827cb4fe62a3 -r e16ca9fde7aa mercurial/upgrade.py
--- a/mercurial/upgrade.py	Mon Sep 09 22:42:55 2019 +0200
+++ b/mercurial/upgrade.py	Wed Sep 04 02:43:17 2019 +0200
@@ -317,6 +317,19 @@
                        'better compression and faster exchange with server.')
 
 @registerformatvariant
+class sidedata(requirementformatvariant):
+    name = 'sidedata'
+
+    _requirement = localrepo.SIDEDATA_REQUIREMENT
+
+    default = False
+
+    description = _('Allows storage of extra data alongside a revision, '
+                    'unlocking various caching options.')
+
+    upgrademessage = _('Allows storage of extra data alongside a revision.')
+
+@registerformatvariant
 class removecldeltachain(formatvariant):
     name = 'plain-cl-delta'