Consider adding a formatter for `std::expected<void, E>`： ```cpp template <typename E, typename Char> struct formatter<std::expected<void, E>, Char, std::enable_if_t<is_formattable<E, Char>::value>> { FMT_CONSTEXPR auto parse(parse_context<Char>& ctx) -> const Char* { return ctx.begin(); } template <typename FormatContext> auto format(const std::expected<void, E>& value, FormatContext& ctx) const -> decltype(ctx.out()) { auto out = ctx.out(); if (value.has_value()) { out = detail::write<Char>(out, "expected("); } else { out = detail::write<Char>(out, "unexpected("); out = detail::write_escaped_alternative<Char>(out, value.error()); } *out++ = ')'; return out; } }; ``` > Output `expected()` when there is no error