Deprecation: #97857 - Deprecate __inheritances operator in form configuration

Description

The custom __inheritances operator, which was available only in YAML configuration files of EXT:form, has been deprecated.

Previously, this operator was used within form definition files to inherit and reuse configuration parts between form element definitions. With native YAML functionality now providing equivalent and more flexible features, this TYPO3-specific operator is no longer necessary.

Developers are encouraged to migrate to standard YAML features such as anchors, aliases, and overrides to avoid code duplication and to simplify form configuration maintenance.

Impact

Using the __inheritances operator inside a custom YAML form configuration in EXT:form will trigger a PHP E_USER_DEPRECATED error.

Affected installations

All installations with custom form definitions or form element configurations that use the __inheritances operator in their EXT:form YAML files are affected and need to update those files accordingly.

Migration

The custom TYPO3 implementation using __inheritances can be replaced with standard YAML syntax.

Developers can achieve the same result by using anchors (&), aliases (*), and overrides (<<:).

Before:

mixins:
  formElementMixins:
    BaseFormElementMixin:
      1761226183:
        identifier: custom
        templateName: Inspector-TextEditor
        label: Custom editor
        propertyPath: custom
    OtherBaseFormElementMixin:
      1761226184:
        identifier: otherCustom
        templateName: Inspector-TextEditor
        label: Other custom editor
        propertyPath: otherCustom

prototypes:
  standard:
    formElementsDefinition:
      Text:
        formEditor:
          editors:
            __inheritances:
              10: 'mixins.formElementMixins.BaseFormElementMixin'
              20: 'mixins.formElementMixins.OtherBaseFormElementMixin'

After:

customEditor: &customEditor
  1761226183:
    identifier: custom
    templateName: Inspector-TextEditor
    label: Custom editor
    propertyPath: custom

otherCustomEditor: &otherCustomEditor
  identifier: otherCustom
  templateName: Inspector-TextEditor
  label: Other custom editor
  propertyPath: otherCustom

prototypes:
  standard:
    formElementsDefinition:
      Text:
        formEditor:
          editors:
            <<: *customEditor
            1761226184: *otherCustomEditor

Inheriting a complete element across files

A common use case of __inheritances was to base a new form element on the complete configuration of an existing (core) element and then override only a few properties. YAML anchors and aliases are file-local and therefore cannot reference a definition from another file. This can instead be solved with the imports key and %...% placeholders of the TYPO3 YAML loader.

Because a placeholder is resolved after parsing and replaces a whole value, the inheritance and the overrides must live in two files: the imported file copies the complete element subtree, the importing file then merges its overrides on top.

Imported file, copies the whole Text element to CustomText:

EXT:my_extension/Configuration/Form/CustomElement/CustomTextInherit.yaml

imports:
  - { resource: 'EXT:form/Configuration/Form/Base/FormElements/Text.yaml' }

prototypes:
  standard:
    formElementsDefinition:
      CustomText: '%prototypes.standard.formElementsDefinition.Text%'

Importing file, overrides only single properties:

EXT:my_extension/Configuration/Form/CustomElement/config.yaml

imports:
  - { resource: 'EXT:my_extension/Configuration/Form/CustomElement/CustomTextInherit.yaml' }

prototypes:
  standard:
    formElementsDefinition:
      CustomText:
        formEditor:
          label: 'Custom Text'
          group: custom

CustomText now inherits the complete configuration of the core Text element (implementationClassName, renderingOptions, all formEditor.editors, predefinedDefaults, …) while only the listed properties are overridden.