Warn that branch does not exist instead of saying that there is

nothing to merge in merge request page

app/views/projects/merge_requests/widget/_open.html.haml

@@ -9,10 +9,10 @@
 
     - if @project.archived?
       = render 'projects/merge_requests/widget/open/archived'
-    - elsif @merge_request.commits.blank?
-      = render 'projects/merge_requests/widget/open/nothing'
     - elsif @merge_request.branch_missing?
       = render 'projects/merge_requests/widget/open/missing_branch'
+    - elsif @merge_request.commits.blank?
+      = render 'projects/merge_requests/widget/open/nothing'
     - elsif @merge_request.unchecked?
       = render 'projects/merge_requests/widget/open/check'
     - elsif @merge_request.cannot_be_merged? && !resolved_conflicts

spec/features/merge_requests/deleted_source_branch_spec.rb

@@ -0,0 +1,30 @@
+require 'spec_helper'
+
+describe 'Deleted source branch', feature: true, js: true do
+  let(:user) { create(:user) }
+  let(:merge_request) { create(:merge_request) }
+
+  before do
+    login_as user
+    merge_request.project.team << [user, :master]
+    merge_request.update!(source_branch: 'this-branch-does-not-exist')
+    visit namespace_project_merge_request_path(
+      merge_request.project.namespace,
+      merge_request.project, merge_request
+    )
+  end
+
+  it 'shows a message about missing source branch' do
+    expect(page).to have_content(
+      'Source branch this-branch-does-not-exist does not exist'
+    )
+  end
+
+  it 'hides Discussion, Commits and Changes tabs' do
+    within '.merge-request-details' do
+      expect(page).to have_no_content('Discussion')
+      expect(page).to have_no_content('Commits')
+      expect(page).to have_no_content('Changes')
+    end
+  end
+end