Answer by HolyBlackCat for How to validate strings to compile time?

The way libfmt (and std::format) do it is by passing a custom class instead of std::string_view, with a consteval constructor accepting a string. Said constructor runs at compile-time and can e.g. throw (or do something else) to cause a compile-time error.

But this approach doesn't let you do this:

create a std::tuple based on the i and ss in the provided string

This requires passing the string as a template parameter:

template <std::size_t N>struct ConstString{    char value[N]{};    constexpr ConstString() {}    constexpr ConstString(const char (&str)[N])    {        std::copy_n(str, N, value);    }    [[nodiscard]] constexpr std::string_view view() const    {        return std::string_view(value, value + N - 1);    }};template <ConstString S>void foo();int main(){    foo<"blah">();}

Or with a slightly different call syntax:

template <ConstString>struct ConstStringTag {};template <ConstString S>[[nodiscard]] ConstStringTag<S> constexpr operator""_c() {return {};}template <ConstString S>void foo(ConstStringTag<S>);int main(){    foo("blah"_c);}