Also inject new route helpers into includers of GitlabRoutingHelper

app/helpers/gitlab_routing_helper.rb

@@ -2,6 +2,10 @@
 module GitlabRoutingHelper
   extend ActiveSupport::Concern
 
+  included do
+    Gitlab::Routing.includes_helpers(self)
+  end
+
   # Project
   def project_tree_path(project, ref = nil, *args)
     namespace_project_tree_path(project.namespace, project, ref || @ref || project.repository.root_ref, *args) # rubocop:disable Cop/ProjectPathHelper

lib/gitlab/routing.rb

@@ -6,21 +6,26 @@ module Gitlab
     self._includers = []
 
     included do
-      Gitlab::Routing._includers << self
+      Gitlab::Routing.includes_helpers(self)
+
       include Gitlab::Routing.url_helpers
     end
 
+    def self.includes_helpers(klass)
+      self._includers << klass
+    end
+
     def self.add_helpers(mod)
       url_helpers.include mod
       url_helpers.extend mod
 
+      GitlabRoutingHelper.include mod
+      GitlabRoutingHelper.extend mod
+
       app_url_helpers = Gitlab::Application.routes.named_routes.url_helpers_module
       app_url_helpers.include mod
       app_url_helpers.extend mod
 
-      GitlabRoutingHelper.include mod
-      GitlabRoutingHelper.extend mod
-
       _includers.each do |klass|
         klass.include mod
       end