diff --git a/gcc/testsuite/g++.dg/warn/string1.C b/gcc/testsuite/g++.dg/warn/string1.C
index a0b0d47de1b..9eb5fb696be 100644
--- a/gcc/testsuite/g++.dg/warn/string1.C
+++ b/gcc/testsuite/g++.dg/warn/string1.C
@@ -18,3 +18,6 @@ int test() {
 
   return 0;
 }
+
+// With -std=c++17 we get another warning deep under operator+=.
+// { dg-prune-output __builtin_memcpy }