The following code compiles and runs fine as long as someFuncTemplate(func1) is commented out. However, once it is uncommented, compilation fails with the error " ‘value’ is not a member of ‘Arity<int (*)(int, int)>’ " compile error, which I believe is due to the compiler selecting the first template instead of the second one at Arity<F>::value. I tried removing the brackets from the first template, but the issue remains, just this time with an "incomplete type" compilation error.

How can I force the compiler to select the correct template when I use Arity<F>?

#include <iostream>

template <typename T>
struct Arity {};

template <typename Ret, typename... Args>
struct Arity<Ret(Args...)>
    : std::integral_constant<size_t, sizeof...(Args)> 
{};

int func1(int a, int b) { return a + b; }

template <typename F>
void someFuncTemplate(F)
{
    std::cout << Arity<F>::value;
}

int main()
{
    std::cout << Arity<decltype(func1)>::value << '\n';
    //someFuncTemplate(func1);
}

>Solution :

You need to add another specialization for the second case:

template <typename Ret, typename... Args>
struct Arity<Ret (*)(Args...)> : Arity<Ret(Args...)> {};
//               ^^^

You could also add

template <typename Ret, typename... Args>
struct Arity<Ret (&)(Args...)> : Arity<Ret(Args...)> {};

in order to make a function declaration such as the below working:

template <typename F>
void someFuncTemplate(F&&) {
    std::cout << Arity<F>::value;
}

Demo