Merge branch '24627-fix-bad-mr-error-message' into 'master'

Fixes regression causing an error message to appear when user first enters the Merge Request form.

Closes #24627

See merge request !7599

app/services/merge_requests/build_service.rb

@@ -48,11 +48,11 @@ module MergeRequests
       end
       # See if source and target branches exist
-      unless merge_request.source_project.commit(merge_request.source_branch)
+      if merge_request.source_branch.present? && !merge_request.source_project.commit(merge_request.source_branch)
         messages << "Source branch \"#{merge_request.source_branch}\" does not exist"
       end
-      unless merge_request.target_project.commit(merge_request.target_branch)
+      if merge_request.target_branch.present? && !merge_request.target_project.commit(merge_request.target_branch)
         messages << "Target branch \"#{merge_request.target_branch}\" does not exist"
       end

changelogs/unreleased/24627-fix-bad-mr-error-message.yml

+---
+title: Fix regression causing bad error message to appear on Merge Request form
+merge_request: 7599
+author: Alex Sanford

spec/features/merge_requests/create_new_mr_spec.rb

@@ -77,4 +77,11 @@ feature 'Create New Merge Request', feature: true, js: true do
       expect(page).to have_content "6049019_460s.jpg"
     end
   end
+
+  # Isolates a regression (see #24627)
+  it 'does not show error messages on initial form' do
+    visit new_namespace_project_merge_request_path(project.namespace, project)
+    expect(page).not_to have_selector('#error_explanation')
+    expect(page).not_to have_content('The form contains the following error')
+  end
 end