Execute quick actions when creating MR from issue

In CE, this does nothing - the `MergeRequests::BuildService` will, at the time
of writing, never return a description for this case.

In EE, a project can have a default MR template, which will be returned by the
service. Previously we were only using the description passed in the params,
ignoring any already set on the object. Now we fall back to the one set on the
object if there was none in the params, allowing quick actions to be executed
from default MR templates when creating an MR from an issue.

app/services/issuable_base_service.rb

@@ -106,12 +106,14 @@ class IssuableBaseService < BaseService
   end
 
   def merge_quick_actions_into_params!(issuable)
+    original_description = params.fetch(:description, issuable.description)
+
     description, command_params =
       QuickActions::InterpretService.new(project, current_user)
-        .execute(params[:description], issuable)
+        .execute(original_description, issuable)
 
     # Avoid a description already set on an issuable to be overwritten by a nil
-    params[:description] = description if params.key?(:description)
+    params[:description] = description if description
 
     params.merge!(command_params)
   end

changelogs/unreleased/fix-create-mr-from-issue-with-template.yml

+---
+title: Execute quick actions (if present) when creating MR from issue
+merge_request: 15810
+author:
+type: fixed

spec/services/merge_requests/create_from_issue_service_spec.rb

@@ -100,5 +100,17 @@ describe MergeRequests::CreateFromIssueService do
 
       expect(result[:merge_request].target_branch).to eq(project.default_branch)
     end
+
+    it 'executes quick actions if the build service sets them in the description' do
+      allow(service).to receive(:merge_request).and_wrap_original do |m, *args|
+        m.call(*args).tap do |merge_request|
+          merge_request.description = "/assign #{user.to_reference}"
+        end
+      end
+
+      result = service.execute
+
+      expect(result[:merge_request].assignee).to eq(user)
+    end
   end
 end