Merge pull request #15971 from maschwenk/mfs-try-to-fix-upstream-issue

Reset requirement cache (again) after `recursive_dependencies.map(&:to_formula)` invalidates singleton cache

Library/Homebrew/requirement.rb

@@ -239,7 +239,13 @@ class Requirement
         end
       end
 
-      cache[cache_key][cache_id dependent] = reqs.dup if cache_key.present?
+      if cache_key.present?
+        # Even though we setup the cache above
+        # 'dependent.recursive_dependencies.map(&:to_formula)'
+        # is invalidating the singleton cache
+        cache[cache_key] ||= {}
+        cache[cache_key][cache_id dependent] = reqs.dup
+      end
       reqs
     end