Create compare object with default branch when target branch is missing

app/services/merge_requests/build_service.rb

@@ -2,18 +2,14 @@ module MergeRequests
   class BuildService < MergeRequests::BaseService
     def execute
       self.merge_request = MergeRequest.new(params)
-      merge_request.can_be_created  = true
       merge_request.compare_commits = []
       merge_request.source_project  = find_source_project
       merge_request.target_project  = find_target_project
       merge_request.target_branch   = find_target_branch
+      merge_request.can_be_created  = branches_valid? && source_branch_specified? && target_branch_specified?
 
-      if branches_specified? && branches_valid?
-        compare_branches
-        assign_title_and_description
-      else
-        merge_request.can_be_created = false
-      end
+      compare_branches if branches_present?
+      assign_title_and_description if merge_request.can_be_created
 
       merge_request
     end
@@ -37,11 +33,17 @@ module MergeRequests
       target_branch || target_project.default_branch
     end
 
-    def branches_specified?
-      params[:source_branch] && params[:target_branch]
+    def source_branch_specified?
+      params[:source_branch].present?
+    end
+
+    def target_branch_specified?
+      params[:target_branch].present?
     end
 
     def branches_valid?
+      return false unless source_branch_specified? || target_branch_specified?
+
       validate_branches
       errors.blank?
     end
@@ -55,9 +57,11 @@ module MergeRequests
         target_branch
       )
 
+      if compare
         merge_request.compare_commits = compare.commits
         merge_request.compare = compare
       end
+    end
 
     def validate_branches
       add_error('You must select source and target branch') unless branches_present?

changelogs/unreleased/issue_28051_2.yml

+---
+title: Use default branch as target_branch when parameter is missing
+merge_request:
+author:

spec/services/merge_requests/build_service_spec.rb

@@ -44,15 +44,14 @@ describe MergeRequests::BuildService, services: true do
       end
     end
 
-    context 'missing target branch' do
-      let(:target_branch) { '' }
+    context 'when target branch is missing' do
+      let(:target_branch) { nil }
+      let(:commits) { Commit.decorate([commit_1], project) }
 
-      it 'forbids the merge request from being created' do
+      it 'creates compare object with target branch as default branch' do
         expect(merge_request.can_be_created).to eq(false)
-      end
-
-      it 'adds an error message to the merge request' do
-        expect(merge_request.errors).to contain_exactly('You must select source and target branch')
+        expect(merge_request.compare).to be_present
+        expect(merge_request.target_branch).to eq(project.default_branch)
       end
     end