add msg.ToString()

include/sp/protocol/message/MessageImplBuilder.h

@@ -37,8 +37,12 @@ struct MessageImplBuilder {
 	static const bool HasValidImpl = InterfaceOptions::HasValid && ImplOptions::HasFieldsImpl;
 	using Base6 = typename MessageImplProcessValidFields<Base5, HasValidImpl>::Type;
 
+	// Provide ToStringImpl() if possible
+	static const bool HasToStringImpl = InterfaceOptions::HasToString;
+	using Base7 = typename MessageImplProcessToString<Base6, ImplOptions, HasToStringImpl>::Type;
+
 	// The last BaseN must be taken as final type.
-	using Type = Base6;
+	using Type = Base7;
 };
 
 }  // namespace details