diff -r 4a170261cc54 -r da87dbdf3daf test/maps_test.cc
--- a/test/maps_test.cc	Thu Feb 25 09:46:12 2021 +0100
+++ b/test/maps_test.cc	Tue Jul 28 21:23:36 2020 +0200
@@ -61,6 +61,9 @@
   typedef A argument_type;
   typedef B result_type;
 
+  F() {};
+  F(const F&) {};
+
   B operator()(const A&) const { return B(); }
 private:
   F& operator=(const F&);