func IsTimeout(err error) bool {
	terr, ok := underlyingError(err).(timeout)
	return ok && terr.Timeout()
}

func underlyingErrorIs(err, target error) bool {
	// Note that this function is not errors.Is:
	// underlyingError only unwraps the specific error-wrapping types
	// that it historically did, not all errors implementing Unwrap().
	err = underlyingError(err)
	if err == target {
		return true
	}