TYPO3 Core

As it looks to me there is still a logical error in the code:

In the beginning of FormEngine->getSingleField_typeInput() you can find:

if (!isset($config['checkbox'])) {
    $config['checkbox'] = '0';
    $checkboxIsset = FALSE;
} else {
    $checkboxIsset = TRUE;
}

So if the user did not define checkbox in his TCA, it is defined from now on (isset() will return TRUE).

A few lines later the validation settings are validated and if nothing is configured there this code is executed:

} else {
    $inputId = uniqid('tceforms-textfield-');
    $cssClasses[] = 'tceforms-textfield';
    if ($checkboxIsset === FALSE) {
        $config['checkbox'] = '';
    }
}

Again, $config['checkbox'] will be set to an empty string (FALSE) but not to NULL.

But finally it will not be evaluated if $config['checkbox'] is TRUE but if it isset() in two places:

$paramsList = '\'' . $PA['itemFormElName'] . '\',\'' . implode(',', $evalList) . '\',\'' . trim($config['is_in']) . '\',' . (isset($config['checkbox']) ? 1 : 0) . ',\'' . $config['checkbox'] . '\'';

and

if (isset($config['checkbox'])) {
    $item .= IconUtility::getSpriteIcon('actions-input-clear', array('tag' => 'a', 'class' => 't3-tceforms-input-clearer', 'onclick' => 'document.getElementById(\'' . $inputId . '\').value=\'\';document.getElementById(\'' . $inputId . '\').focus();' . implode('', $PA['fieldChangeFunc'])));
}

So basically currently the checkbox (clear button) is always rendered no matter what the user configured because $config['checkbox'] is always NOT NULL but TRUE or FALSE.

This is why I would vote for getting rid of the setting.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/44129