递归可变参数模板插入失败

Question

What is the reason, that in the following example the instantiaton with three int is working with the second function template but not with the first one. Additional: with the types State1 / State1 it isn't working either.

struct State1{};
struct State2{};

#if 0
template<typename none = void>
constexpr void f()
{
}
template<typename First, typename... Rest>
constexpr void f()
{
    f<Rest...>();
}
#else
template<typename none = void>
constexpr void f()
{
}
template<int First, int... Rest>
constexpr void f()
{
    f<Rest...>();
}
#endif
void test()
{
   f<1, 2, 3>();
//    f<State1, State2>();
}

Thank you any hints!

Answer 1

The problem in the first case is that both template specializations match when you pass a single template parameter (a parameter pack can be empty), so the overload set contains more than one candidate. You can use SFINAE to make the second one fail to instantiate:

#include <type_traits>

struct State1{};
struct State2{};

template<typename none = void>
constexpr auto f() -> void
{
}

template<typename First, typename... Rest>
constexpr auto f() -> std::enable_if<sizeof...(Rest), void>::type
    // This will not instantiate when the parameter pack is empty
    // And the overload set will contain only the candidate above
{
    f<Rest...>();
}

int main()
{
  f<State1, State2>();

  return 0;
}