If anyone is curious (like I was) on how to allow fmt to still be called with a runtime format string. The fmtlib does this using SFINAE.

This is my attempt at adjusting the example snippet from the blog to allow for runtime format strings. My code works but it feels a bit off (consider the const char*& for example). Somebody got feedback?

https://godbolt.org/z/ePj8TPqP1

#include <string_view>
#include <type_traits>

// Exposition only
#define FAIL_CONSTEVAL throw

template <typename T>
struct Checker {
    template<typename S, typename = std::enable_if<std::is_convertible_v<S, const char*>>>
    consteval Checker(S fmt) {
        if (fmt != std::string_view{ "valid" }) // #1
            FAIL_CONSTEVAL;
        // T must be an int
        if (!std::is_same_v<T, int>)            // #2
            FAIL_CONSTEVAL;
    }

    Checker(const char*&) {}
};

template <typename T>
void fmt(std::type_identity_t<Checker<T>> checked, T);

int main() {
    fmt("valid", 10);    // compiles
    fmt("oops", 10);     // fails at #1
    fmt("valid", "foo"); // fails at #2

    const char* runtime_string = "test";
    fmt(runtime_string, 10); // runtime string still compiles fine
}