Fix regression in Merge request form

Merge request !7163 caused an error message to appear on the Merge
Request form when it was first loaded by the user. This commit fixes
that regression bug.

3 files changed, 13 insertions, 2 deletions

diff --git a/app/services/merge_requests/build_service.rb b/app/services/merge_requests/build_service.rb
index f415244068b..dd0d738674e 100644
--- a/app/services/merge_requests/build_service.rb
+++ b/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
 
diff --git a/changelogs/unreleased/24627-fix-bad-mr-error-message.yml b/changelogs/unreleased/24627-fix-bad-mr-error-message.yml
new file mode 100644
index 00000000000..d6a9818b2ce
--- /dev/null
+++ b/changelogs/unreleased/24627-fix-bad-mr-error-message.yml
@@ -0,0 +1,4 @@
+---
+title: Fix regression causing bad error message to appear on Merge Request form
+merge_request: 7599
+author: Alex Sanford
diff --git a/spec/features/merge_requests/create_new_mr_spec.rb b/spec/features/merge_requests/create_new_mr_spec.rb
index 584574cc91a..702869b6e8b 100644
--- a/spec/features/merge_requests/create_new_mr_spec.rb
+++ b/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