TYPO3 Core

1. Story
   As a developer i want to be able to use a database independent treeConfig/DataProvider in TCA/FlexForms select fields of renderType selectTree.

1. Details
   ATM the TreeDataProviderFactory is tightly coupled to the internal implementation of the DatabaseTreeDataProvider.
   It checks for 'foreign_table', 'childrenField' and 'parentField' while this should be done by the specific TreeDataProvider itself.

With the removal of the "internal_type" check this behavior forces you to set thoose fields even if your treeDataProvide is not even using a database see:
https://github.com/TYPO3/typo3/commit/6f557ac5aae1b44f882dfbe09d60c2f68ce3534d#diff-2a75aeb556ea1fbf95b8ff85c45804ae0141c922fe2b1561362fa91555ce0c40L69

1. Example
   Lets say you want to provide TreeData from remote Source e.g. a Webservice.
   In this case you need to provide dummy foreign_table, ChildrenField and ParentField as well as adding the corresponding setters to your TreeDataProvider even though they are not used at all.

1. Solution
   Leave the TCA Checks to the Dataprovider itself.
   Remove the tight coupling to the DatabaseTreeProvider:
   https://github.com/TYPO3/typo3/blob/main/typo3/sysext/core/Classes/Tree/TableConfiguration/TreeDataProviderFactory.php#L71
   https://github.com/TYPO3/typo3/blob/main/typo3/sysext/core/Classes/Tree/TableConfiguration/TreeDataProviderFactory.php#L73
   https://github.com/TYPO3/typo3/blob/main/typo3/sysext/core/Classes/Tree/TableConfiguration/TreeDataProviderFactory.php#L80
   https://github.com/TYPO3/typo3/blob/main/typo3/sysext/core/Classes/Tree/TableConfiguration/TreeDataProviderFactory.php#L112

A treeConfig with a custom TreeDataProvider is not "invalid" if "foreign_table" or the child/Parent field is missing :)