If you want to have your own Base32 where giving a padding = 0xFF instead returns an error, it's commonplace to have a wrapper that will explicitly do the check and return an error if it fails, and otherwise call the function:

    macro String? encode2(Allocator allocator, char[] src, char padding = DEFAULT_PAD, Base32Alphabet* alphabet = &STANDARD)
    {
        if (padding >= 0xFF) return INVALID_PADDING?;
        return encode(allocator, src, padding, alphabet);
    }

Maybe a better place to ask these questions are on the Discord if you have an account: https://discord.gg/qN76R87