std::apply and constant expression?

Question

I tried code below in Wandbox:

#include <array>
#include <iostream>
#include <tuple>
#include <typeinfo>
#include <functional>
#include <utility>


int main()
{
    constexpr std::array<const char, 10> str{"123456789"};
    constexpr auto foo = std::apply([](auto... args) constexpr { std::integer_sequence<char, args...>{}; } , str);
    std::cout << typeid(foo).name();
}

and the compiler told me that args... are not constant expression. What's wrong?

Answer 1

Function parameters cannot be labeled constexpr. As such, you cannot use them in places that require constant expressions, like non-type template arguments.

To do the kind of thing you're trying to do would require some kind of compile-time string processing, based around template arguments.