client: implement `interface{ Is(error) bool }` on generated errors

[kushuh]

Problem

The template for generating errors only implements the Error() string interface.

Since the generated error is a struct, errors.Is() cannot be used directly. The only way to check errors is to use the more cumbersome errors.As() alternative.

_, err := client.Method(ctx)

var ogenError *generated.UnexpectedErrorStatusCode

if errors.As(err, &ogenError) {
  if ogenError.StatusCode == http.StatusUnauthorized {
    // ...
  }
}

Proposal

Golang allows errors to implement the interface{ Is(error) bool }, which allows complex errors to be scanned with errors.Is. Something as straightforward as:

// Code generated by ogen.

func (s *UnexpectedErrorStatusCode) Error() string {
  return fmt.Sprintf("code %d: %+v", s.StatusCode, s.Response)
}

func (s *UnexpectedErrorStatusCode) Is(err error) bool {
  var cmp *UnexpectedErrorStatusCode

  if !errors.As(err, &cmp) {
    return false
  }

  if cmp.StatusCode != s.StatusCode {
    return false
  }

  return true
}

Could allow one-line checks like this:

if errors.Is(err, &generated.UnexpectedErrorStatusCode{StatusCode: http.StatusUnauthorized}) {}

Or even allow for nice switch statements:

unauthorizedError := &generated.UnexpectedErrorStatusCode{StatusCode: http.StatusUnauthorized}
forbiddenError := &generated.UnexpectedErrorStatusCode{StatusCode: http.StatusForbidden}

switch {
case errors.Is(err, unauthorizedError):
  // Unauthorized.
case errors.Is(err, forbiddenError):
  // Forbidden.
default:
  // Unhandled.
}

References

This feature would be insanely helpful for integration tests. Also it could improve code quality on applications that use the ogen client.