template parameter pack in using

Question

How i can use "using" keyword in multiple inheritance when parameter pack is template parameter of base class?

Code below compiles just fine

struct A
{
 void foo(int) {}
};
struct B
{
 void foo(int) {}
};
template<typename ...Types>
struct C : public Types...
{
 using Types::foo...;
};
int main()
{
 C<A,B> c;
}

But if i use template instead of A and B - I've got compile error

template<typename T> 
struct TA {};
template<>
struct TA<int>
{ 
 void foo(int) {} 
};
template<>
struct TA<double>
{
 void foo(int) {}
};
template<typename ...Types>
struct TC : public TA<Types>...
{
 using TA<Types>::foo...; // ERROR C3520
};

Error:

error C3520: 'Types': parameter pack must be expanded in this context

How to rewrite second piece of code to get it work?

PS I tried this code with gcc and it's compiles without errors. But now I am using msvc...

• François Andrieux: Visual Studio has a persisting problem with unpacking parameter packs in some circumstances.
• Jarod42: Related to variadic-base-class-using-declaration-fails-to-compile-in-msvc

Answer 1

Since it's a known MSVC bug, if you still want to make this work with MSVC you'll have to do it the (not very) hard way:

template <typename ...Types>
struct TC;
template <typename T>
struct TC<T> : TA<T>
{
 using TA<T>::foo;
};
template <typename T, typename ...Types>
struct TC<T, Types...> : public TC<T>, public TC<Types...>
{
 using TC<T>::foo;
 using TC<Types...>::foo;
};

• L. F.: In fact, this is how things have to be done prior to C++17