silverstripe-userforms/code/model/editableformfields/EditableFormFieldValidator.php

<?php

class EditableFormFieldValidator extends RequiredFields {

	/**
	 *
	 * @var EditableFormField
	 */
	protected $record = null;

	/**
	 *
	 * @param EditableFormField $record
	 * @return $this
	 */
	public function setRecord($record) {
		$this->record = $record;
		return $this;
	}

	/*
	 * @return EditableFormField
	 */
	public function getRecord() {
		return $this->record;
	}


	public function php($data) {
		if(!parent::php($data)) {
			return false;
		}

		// Skip unsaved records
		if(!$this->record || !$this->record->exists()) {
			return true;
		}

		// Skip validation if not required
		if(empty($data['Required'])) {
			return;
		}

		// Skip validation if no rules
		$count = EditableCustomRule::get()->filter('ParentID', $this->record->ID)->count();
		if($count == 0) {
			return true;
		}

		// Both required = true and rules > 0 should error
		$this->validationError(
			'Required_Error',
			_t(
				"EditableFormFieldValidator.REQUIRED_ERROR",
				"Form fields cannot be required and have conditional display rules."
			),
			'error'
		);
		return false;
	}
}