Problem/Motivation
In core we've held off from using final on classes because of the idea that final limits extensibility and a key feature is extensibility.
However there are cases where we've defined specific types of classes in core as not being extensible and @internal by default. These include:
- Plugins
- Events
- Forms
- Controllers
- Paramconvertors
See https://www.drupal.org/core/d8-bc-policy
However people are extending these in contrib and custom code and then when we make changes to the constructors things break. For example:
https://www.drupal.org/project/drupal/issues/2594425 and then https://www.drupal.org/project/menu_block/issues/2968049
Proposed resolution
Use final instead of an implicit @internal. Put in another way we need to prioritise composition over inheritance. So a plugin that wants to borrow functionality from another plugin could create that plugin in it's constructor and then use that object. See #2968049-18: Dependency Injection issue as an example.
Making this change is likely to shift some costs from future maintenance to the here-and-now but in the long run it means that core upgrades to major and minor releases can be done with more confidence because our conventions have moved from wishful thinking to language constructs.
Remaining tasks
- Bring in the technical working group as this is likely to involve coding standards.
- Build core consensus - this is likely to be tricky because there is an idea that the use of
finalis bad.
User interface changes
None
API changes
None :) everything is already implicitly @internal
Data model changes
None
Release notes snippet
This is Drupal 9 only change.
