Provide explicit copy operator declaration to avoid compiler warning

peglib.h

@@ -345,6 +345,8 @@ public:
     template <typename F, typename std::enable_if<std::is_same<F, std::nullptr_t>::value>::type*& = enabler>
     void operator=(F /*fn*/) {}
 
+    Action& operator=(const Action& rhs) = default;
+
     operator bool() const {
         return (bool)fn_;
     }