diff -Nrup a/external/thp/thp_format.h b/external/thp/thp_format.h
--- a/external/thp/thp_format.h	2015-12-31 01:38:46.000000000 -0700
+++ b/external/thp/thp_format.h	2019-09-23 16:09:43.982154904 -0600
@@ -17,6 +17,7 @@
 #define THP_FORMAT_H
 
 #include <functional>
+#include <string>
 
 namespace thp {