to_string() doesn't work with custom format_as()?

[usefulcat]

I thought that fmt::to_string() would use custom format_as() overloads, but it seems not to. Is this expected?

Using gcc13, c++20
https://godbolt.org/z/3fo53PoYW

#include <cassert>
#include <fmt/format.h>

enum class MyEnum : char {
    one = '1',
    two = '2',
    three = '3'
};

char format_as(MyEnum e) {
    return static_cast<char>(e);
}

int main() {
    assert(fmt::format("{}", MyEnum::one) == "1"); // compiles and works as expected
    assert(fmt::to_string(MyEnum::one) == "1");    // does not compile
    return 0;
}

[vitaut]

This is not supported at the moment but a PR is welcome.