C3520 parameter pack must be expanded - incorrect behaviour for 'variadic using'-CodePudding

While compiling the code below with Qt 5.12 using Microsoft Visual C Compiler 15.9.28307.1300 (amd64) and c 17 standard I get the following error:

error C3520: 'Args': parameter pack must be expanded in this context note: see reference to class template instantiation 'Helper<Args...>' being compiled

template<typename T> 
struct Base { 
    void operator()(const T& arg){} 
}; 

template <typename... Args>
class Helper : Base<Args>... {
public:
    using Base<Args>::operator()...;
};

Is this a bug with msvc? Is there any known workaround? The same code compiles on macOS using clang.

CodePudding user response：

As workaround to variadic using (C 17), you might use the recursive way:

template <typename... Args>
class Helper;

template <>
class Helper<>
{
};

template <typename T>
class Helper<T> : Base<T>
{
public:
    using Base<T>::operator();
};

template <typename T, typename... Ts>
class Helper<T, Ts...> : Base<T>, Helper<Ts...>
{
public:
    using Base<T>::operator();
    using Helper<Ts...>::operator();
};

Demo