Respect the should_remove_source_branch parameter to the merge API

changelogs/unreleased/58020-fix-merge-api-endpoint-param.yml

+---
+title: Respect the should_remove_source_branch parameter to the merge API
+merge_request: 25525
+author:
+type: fixed

lib/api/merge_requests.rb

@@ -369,11 +369,11 @@ module API
 
         merge_request.update(squash: params[:squash]) if params[:squash]
 
-        merge_params = {
+        merge_params = HashWithIndifferentAccess.new(
           commit_message: params[:merge_commit_message],
           squash_commit_message: params[:squash_commit_message],
           should_remove_source_branch: params[:should_remove_source_branch]
-        }
+        )
 
         if merge_when_pipeline_succeeds && merge_request.head_pipeline && merge_request.head_pipeline.active?
           ::MergeRequests::MergeWhenPipelineSucceedsService

spec/requests/api/merge_requests_spec.rb

@@ -984,6 +984,21 @@ describe API::MergeRequests do
         expect(squash_commit.message).to eq(merge_request.default_squash_commit_message)
       end
     end
+
+    describe "the should_remove_source_branch param" do
+      let(:source_repository) { merge_request.source_project.repository }
+      let(:source_branch) { merge_request.source_branch }
+
+      it 'removes the source branch when set' do
+        put(
+          api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/merge", user),
+          params: { should_remove_source_branch: true }
+        )
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(source_repository.branch_exists?(source_branch)).to be_falsy
+      end
+    end
   end
 
   describe "PUT /projects/:id/merge_requests/:merge_request_iid" do