Number::validateNumber

public static Number::validateNumber(&$element, FormStateInterface $form_state, &$complete_form)

public static function validateNumber(&$element, FormStateInterface $form_state, &$complete_form) {
  $value = $element['#value'];
  if ($value === '') {
    return;
  }

  $name = empty($element['#title']) ? $element['#parents'][0] : $element['#title'];

  // Ensure the input is numeric.
  if (!is_numeric($value)) {
    $form_state->setError($element, t('%name must be a number.', array('%name' => $name)));
    return;
  }

  // Ensure that the input is greater than the #min property, if set.
  if (isset($element['#min']) && $value < $element['#min']) {
    $form_state->setError($element, t('%name must be higher than or equal to %min.', array('%name' => $name, '%min' => $element['#min'])));
  }

  // Ensure that the input is less than the #max property, if set.
  if (isset($element['#max']) && $value > $element['#max']) {
    $form_state->setError($element, t('%name must be lower than or equal to %max.', array('%name' => $name, '%max' => $element['#max'])));
  }

  if (isset($element['#step']) && strtolower($element['#step']) != 'any') {
    // Check that the input is an allowed multiple of #step (offset by #min if
    // #min is set).
    $offset = isset($element['#min']) ? $element['#min'] : 0.0;

    if (!NumberUtility::validStep($value, $element['#step'], $offset)) {
      $form_state->setError($element, t('%name is not a valid number.', array('%name' => $name)));
    }
  }
}