Fix bug preventing mentioned issued from being closed when MR is merged using fast-forward merge.

CHANGELOG

@@ -5,6 +5,7 @@ v 8.2.0 (unreleased)
   - Highlight comment based on anchor in URL
 
 v 8.1.0 (unreleased)
+  - Fix bug preventing mentioned issued from being closed when MR is merged using fast-forward merge.
   - Fix nonatomic database update potentially causing project star counts to go negative (Stan Hu)
   - Fix error preventing displaying of commit data for a directory with a leading dot (Stan Hu)
   - Speed up load times of issue detail pages by roughly 1.5x

app/services/merge_requests/post_merge_service.rb

@@ -6,6 +6,7 @@ module MergeRequests
   #
   class PostMergeService < MergeRequests::BaseService
     def execute(merge_request)
+      close_issues(merge_request)
       merge_request.mark_as_merged
       create_merge_event(merge_request, current_user)
       create_note(merge_request)
@@ -15,6 +16,15 @@ module MergeRequests
 
     private
 
+    def close_issues(merge_request)
+      return unless merge_request.target_branch == project.default_branch
+
+      closed_issues = merge_request.closes_issues(current_user)
+      closed_issues.each do |issue|
+        Issues::CloseService.new(project, current_user, {}).execute(issue, merge_request)
+      end
+    end
+
     def create_merge_event(merge_request, current_user)
       EventCreateService.new.merge_mr(merge_request, current_user)
     end