Breaking: #108148 - Disallow Fluid Variable Names With Underscore Prefix 

Description 

With Fluid 5, it is no longer possible to define custom template variables that start with an underscore (_). These variable names are reserved for future internal use by Fluid itself, similarly to the already existing {_all}.

Impact 

This change affects ViewHelpers that define new variables, such as <f:variable>, <f:for> or <f:render>. It also affects Fluid's PHP APIs, namely $view->assign() and $view->assignMultiple().

Affected installations 

Installations with Fluid templates that use custom variable names starting with an underscore (_) will encounter exceptions when such a template is rendered. A deprecation is written to the deprecation log since TYPO3 13.4.21 if this is encountered in a Fluid template during rendering.

Migration 

The following examples no longer work with Fluid 5:

<f:variable name="_temp" value="a temporary value" />
{_temp}

<f:for each="{myArray}" as="_item">
    {_item}
</f:for>

<f:render partial="Footer" arguments="{_data: myData}" />

$view->assign('_data', $myData);
$view->assignMultiple([
    '_data' => $myData,
]);

All examples would lead to the following exception:

#1756622558 TYPO3Fluid\Fluid\Core\Variables\InvalidVariableIdentifierException
Variable identifiers cannot start with a "_": _myVariable

In all cases, the variable name must be changed to no longer start with an underscore (_).

Note that this only affects variable names, but not property names in objects or array keys that are accessed within a Fluid template. The following examples are not affected by this change:

{myArray._myKey}
{myObject._myProperty}

Also note that the existing {_all} (and any further internal variables added by Fluid) are not affected. This code will continue to work just fine:

<f:render partial="Footer" arguments="{_all}"/>