https://bugs.gentoo.org/768339
--- a/src/mongo/db/exec/plan_stats.h
+++ b/src/mongo/db/exec/plan_stats.h
@@ -33,6 +33,7 @@
#include <cstdlib>
#include <string>
#include <vector>
+#include <optional>
#include "mongo/db/index/multikey_paths.h"