How to use if constexpr in template fold expressions?

As we do not have a ternary constexpr operator, we can use a lambda instead.

#include <type_traits>

template<typename... T>
constexpr auto fold_sum(T... s){
    return (... + [](auto x)
    {
        if constexpr(std::is_arithmetic_v<T>) return x;
        else return 0;
    }(s));
}

Usage:

int main()
{
    static_assert(fold_sum(0, nullptr, 5, nullptr, 11, nullptr) == 16);
}

live example on godbolt.org

You absolutely want to use if constexpr?

I propose a different alternative: std::get() and std::pair to simulate a constexpr ternary operator as follows (with a Vittorio Romeo's improvement; thanks)

#include <utility>
#include <type_traits>

template<typename ... Ts>
constexpr auto fold_sum (Ts const & ... s)
 { return (... + std::get<std::is_arithmetic_v<Ts>>(std::pair{0, s})); }

int main ()
 {
   static_assert(fold_sum(0, nullptr, 5, nullptr, 11, nullptr) == 16);
 }