introduce MergeRequest#issues_mentioned_but_not_closing

app/models/merge_request.rb

@@ -568,6 +568,22 @@ class MergeRequest < ActiveRecord::Base
     end
   end
 
+  def issues_mentioned_but_not_closing(current_user = self.author)
+    issues = []
+
+    if target_branch == project.default_branch
+      messages = [description]
+      messages.concat(commits.map(&:safe_message)) if merge_request_diff
+
+      ext = Gitlab::ReferenceExtractor.new(project, current_user)
+      ext.analyze(messages.join("\n"))
+
+      issues = ext.issues
+    end
+
+    issues - closes_issues
+  end
+
   def target_project_path
     if target_project
       target_project.path_with_namespace

spec/models/merge_request_spec.rb

@@ -252,7 +252,7 @@ describe MergeRequest, models: true do
     end
   end
 
-  describe 'detection of issues to be closed' do
+  describe 'detection of issues' do
     let(:issue0) { create :issue, project: subject.project }
     let(:issue1) { create :issue, project: subject.project }
 
@@ -265,6 +265,7 @@ describe MergeRequest, models: true do
       allow(subject).to receive(:commits).and_return([commit0, commit1, commit2])
     end
 
+    describe 'detection of issues to be closed' do
       it 'accesses the set of issues that will be closed on acceptance' do
         allow(subject.project).to receive(:default_branch).
           and_return(subject.target_branch)
@@ -283,7 +284,9 @@ describe MergeRequest, models: true do
 
       it 'detects issues mentioned in the description' do
         issue2 = create(:issue, project: subject.project)
+
         subject.description = "Closes #{issue2.to_reference}"
+
         allow(subject.project).to receive(:default_branch).
           and_return(subject.target_branch)
 
@@ -291,6 +294,18 @@ describe MergeRequest, models: true do
       end
     end
 
+    it 'detects issues mentioned but not closed' do
+      mentioned_issue = create(:issue, project: subject.project)
+
+      subject.description = "Is related to #{mentioned_issue.to_reference}"
+
+      allow(subject.project).to receive(:default_branch).
+        and_return(subject.target_branch)
+
+      expect(subject.issues_mentioned_but_not_closing).to match_array([mentioned_issue])
+    end
+  end
+
   describe "#work_in_progress?" do
     ['WIP ', 'WIP:', 'WIP: ', '[WIP]', '[WIP] ', ' [WIP] WIP [WIP] WIP: WIP '].each do |wip_prefix|
       it "detects the '#{wip_prefix}' prefix" do