Allow extensible mention type action for EE

app/services/notification_recipient_service.rb

@@ -220,6 +220,10 @@ module NotificationRecipientService
     end
 
     class Default < Base
+      prepend ::EE::NotificationRecipientBuilders::Default
+
+      MENTION_TYPE_ACTIONS = [:new_issue, :new_merge_request].freeze
+
       attr_reader :target
       attr_reader :current_user
       attr_reader :action
@@ -252,7 +256,7 @@ module NotificationRecipientService
 
         add_subscribed_users
 
-        if [:new_issue, :new_merge_request].include?(custom_action)
+        if self.class.mention_type_actions.include?(custom_action)
           # These will all be participants as well, but adding with the :mention
           # type ensures that users with the mention notification level will
           # receive them, too.
@@ -283,6 +287,10 @@ module NotificationRecipientService
       def custom_action
         @custom_action ||= "#{action}_#{target.class.model_name.name.underscore}".to_sym
       end
+
+      def self.mention_type_actions
+        MENTION_TYPE_ACTIONS.dup
+      end
     end
 
     class NewNote < Base

ee/app/services/ee/notification_recipient_builders/default.rb

+# frozen_string_literal: true
+
+module EE
+  module NotificationRecipientBuilders
+    module Default
+      extend ActiveSupport::Concern
+
+      class_methods do
+        extend ::Gitlab::Utils::Override
+
+        override :mention_type_actions
+        def mention_type_actions
+          super.append(:new_epic)
+        end
+      end
+    end
+  end
+end