[TASK] Fluid Viewhelpers will be declared final in v12

Communication patch for an upcoming v12 change.

We've had the situation a couple of times that core
changed Fluid Viewhelpers to adapt them to current
needs, keeping the template arguments, but changing
internal handling and method signatures.

It happened that developers then stepped up with
"This broke my own Viewhelper!" and we had to act
upon. The ImageViewHelper for instance has been a
victim of this.

From a framework point of view, Viewhelpers are
specific implementations tailored for specific use.
In most cases, they're small codewise. Extending
these classes and changing their scope should not
be supported. Developers should extend a matching
abstract instead of extending specific VH's.

The patch brings an Important.rst to pitch an
upcoming v12 change that will declare all Viewhelpers
final. The core currently of course also extends
a couple of VH's itself, but this is too
late for v11 now to change and adapt. Instead,
we'll take the time in v12 to resolve these cases
and will have a look at the abstract classes as
well to see which VH functionality should be
included in abstracts to simplify re-use and
transition for extension authors and to sharpen
core API.

Resolves: #95298
Releases: master
Change-Id: Ia3312829fad94e7429c0989f3aa3554c3cf944ec
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71164


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/core/Documentation/Changelog/master/Important-95298-FluidViewhelpersWillBeDeclaredFinalInV12.rst

+.. include:: ../../Includes.txt
+
+===================================================================
+Important: #95298 - Fluid Viewhelpers will be declared final in v12
+===================================================================
+
+See :issue:`95298`
+
+Description
+===========
+
+This is a notice for an upcoming change in TYPO3 v12:
+
+All Fluid Viewhelper classes delivered by core extensions will be declared
+:php:`final` in TYPO3 v12, third party extensions can no longer extend them
+with own variants.
+
+The core takes this step to clarify that single Viewhelpers are not part of the
+PHP API, their internal handling may change any time, which is not considered
+breaking. Fluid delivers a series of abstract classes to provide base functionality
+for common Viewhelper needs. Those can be used by third party viewhelpers if
+not marked :php:`@internal`. TYPO3 v12 will fine-tune these abstracts and may
+extract specific Viewhelper code to abstracts if the code is generally useful
+for extension developers with own View Helpers.
+
+Using Viewhelpers provided by core extensions in Fluid templates is of course
+fine as long as they are not marked :php:`@internal`. Arguments to casual Viewhelpers
+are considered API and are subject of the general core deprecation strategy. In
+general, the base extensions `EXT:fluid`, `EXT:core`, `EXT:frontend` and `EXT:backend`
+deliver various general purpose Viewhelpers that can be used, while specific extensions
+like `EXT:beuser` add :php:`@internal` Viewhelpers that should not be used in own templates.
+
+Developers are encouraged to adapt own Viewhelpers towards this change with
+v11 compatible extensions already, it will simplify compatibility with v12 later.
+
+.. index:: Fluid, PHP-API, ext:fluid