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...

Visual Studio has a persisting problem with unpacking parameter packs in some circumstances. — François Andrieux, Aug 19 '19 at 17:22
Related to [variadic-base-class-using-declaration-fails-to-compile-in-msvc](https://stackoverflow.com/questions/57554389/variadic-base-class-using-declaration-fails-to-compile-in-msvc) — Jarod42, Aug 19 '19 at 18:24

score 1 · Accepted Answer · answered Aug 19 '19 at 17:43

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;
};

answered Aug 19 '19 at 17:43

r3mus n0x

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In fact, this is how things have to be done prior to C++17 – L. F. Aug 19 '19 at 17:52

template parameter pack in using

1 Answers1