Return more consistent values for merge_status on MR API

changelogs/unreleased/osw-updates-merge-status-on-api-actions.yml

+---
+title: Return more consistent values for merge_status on MR APIs
+merge_request:
+author:
+type: fixed

lib/api/entities.rb

@@ -507,7 +507,14 @@ module API
       expose :work_in_progress?, as: :work_in_progress
       expose :milestone, using: Entities::Milestone
       expose :merge_when_pipeline_succeeds
-      expose :merge_status
+
+      # Ideally we should deprecate `MergeRequest#merge_status` exposure and
+      # use `MergeRequest#mergeable?` instead (boolean).
+      # See https://gitlab.com/gitlab-org/gitlab-ce/issues/42344 for more
+      # information.
+      expose :merge_status do |merge_request|
+        merge_request.tap(&:check_if_can_be_merged).merge_status
+      end
       expose :diff_head_sha, as: :sha
       expose :merge_commit_sha
       expose :user_notes_count

spec/requests/api/merge_requests_spec.rb

@@ -198,6 +198,8 @@ describe API::MergeRequests do
 
         create(:merge_request, state: 'closed', milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Test", created_at: base_time)
 
+        create(:merge_request, milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Test", created_at: base_time)
+
         expect do
           get api("/projects/#{project.id}/merge_requests", user)
         end.not_to exceed_query_limit(control)