inherit signal in libsourcey lib

I'm using libsourcey lib for learning. I try to write a new class that inherit a class named Signal.

in libsourcey

template <typename RT> class Signal;
template <typename RT, typename... Args> class Signal<RT(Args...)> {...}

and my way

template <typename RT> class NewSignal;
template <typename RT, typename... Args> class NewSignal<RT(Args...)> : public Signal<RT(Args...)> {...}

but i can't access any property of class Signal, also attribute (although I modify attribute to public). Complier (eclipse) report: was not declared in this scope

Anyone explain why complier report like this. How i can inherit Signal class. If not, have any solution? Thank for supporting.