Leave it like the old code
@@ -218,7 +218,9 @@ class ResolverBase:
# Track what projects have already been traversed to avoid infinite
# recursion. We can't determine a root project well here, so you get
# what you get if you have configured your project in a strange manner
- projects = projects or set()
+ if projects is None:
+ projects = {project}
+ else:
projects.add(project)
next_project = None
