If you remember the tutorial section on the value_or_error Concept, this is an example of how to implement a custom value_or_error Concept converter in Outcome:

// Inject custom ValueOrError conversion
BOOST_OUTCOME_V2_NAMESPACE_BEGIN
namespace convert
{
  // Provide custom ValueOrError conversion from
  // httplib::result<U> into any app::outcome<T>
  template <class T, class U>  //
  struct value_or_error<app::outcome<T>, httplib::result<U>>
  {
    // False to indicate that this converter wants `result`/`outcome`
    // to NOT reject all other `result`
    static constexpr bool enable_result_inputs = true;
    // False to indicate that this converter wants `outcome` to NOT
    // reject all other `outcome`
    static constexpr bool enable_outcome_inputs = true;

    template <class X,                                                                              //
              typename = std::enable_if_t<std::is_same<httplib::result<U>, std::decay_t<X>>::value  //
                                          && std::is_constructible<T, U>::value>>                   //
    constexpr app::outcome<T> operator()(X &&src)
    {
      // Forward any successful value, else synthesise an exception ptr
      return src.has_value() ?                              //
             app::outcome<T>{std::forward<X>(src).value()}  //
             :
             app::outcome<T>{app::make_httplib_exception(std::forward<X>(src))};
    }
  };
}  // namespace convert
BOOST_OUTCOME_V2_NAMESPACE_END

The first thing that you should note is that these custom converters must be injected directly into the BOOST_OUTCOME_V2_NAMESPACE::convert namespace, and they must partially or completely specialise value_or_error<T, U> . Here we specialise the converter for value_or_error conversions from httplib::result<U> to app::outcome<T> i.e. from our third party HTTP library’s error type into our application’s outcome type (which is unique to our application, as we hard code an app-local error type).

The second thing to note is that you need to set enable_result_inputs and enable_outcome_inputs appropriately, otherwise result and outcome inputs will not be matched by this converter¹. In this converter, we really do wish to convert other result and outcome inputs, so we mark these booleans as true.

The third thing to note is the requirements on operator(). If the requirements are not met, the value_or_error converting constructor in basic_result and basic_outcome disables. Note the requirement that the decayed operator() input X matches httplib::result<U>, and that T is constructible from U. This means that the explicit basic_result(concepts::value_or_error<T, E> &&) and explicit basic_outcome(concepts::value_or_error<T, E> &&) constructors are available if, and only if, the input type is a httplib::result<U>, and the result’s value type is constructible from the input’s value type.

If operator() is available, it naturally converts a httplib::result<U> into an app::outcome<T> by either forwarding any success as-is, or calling app::make_httplib_exception() to type erase the httplib::failure into an app::httplib_error.