22 files declare their use of WidgetBase

BooleanCheckboxWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/BooleanCheckboxWidget.php
CommentWidget.php in core/modules/comment/src/Plugin/Field/FieldWidget/CommentWidget.php
DateTimeWidgetBase.php in core/modules/datetime/src/Plugin/Field/FieldWidget/DateTimeWidgetBase.php
DummyAjaxWidget.php in core/modules/image/tests/modules/image_module_test/src/Plugin/Field/FieldWidget/DummyAjaxWidget.php
EmailDefaultWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/EmailDefaultWidget.php
EntityReferenceAutocompleteWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/EntityReferenceAutocompleteWidget.php
FileWidget.php in core/modules/file/src/Plugin/Field/FieldWidget/FileWidget.php
LanguageSelectWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/LanguageSelectWidget.php
LayoutBuilderWidget.php in core/modules/layout_builder/src/Plugin/Field/FieldWidget/LayoutBuilderWidget.php
LinkWidget.php in core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php
MediaLibraryWidget.php in core/modules/media_library/src/Plugin/Field/FieldWidget/MediaLibraryWidget.php
NumberWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/NumberWidget.php
OptionsWidgetBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php
PathWidget.php in core/modules/path/src/Plugin/Field/FieldWidget/PathWidget.php
ShapeOnlyColorEditableWidget.php in core/modules/system/tests/modules/entity_test/src/Plugin/Field/FieldWidget/ShapeOnlyColorEditableWidget.php
StringTextareaWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/StringTextareaWidget.php
StringTextfieldWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/StringTextfieldWidget.php
TelephoneDefaultWidget.php in core/modules/telephone/src/Plugin/Field/FieldWidget/TelephoneDefaultWidget.php
TestFieldWidget.php in core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldWidget/TestFieldWidget.php
TestFieldWidgetMultiple.php in core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldWidget/TestFieldWidgetMultiple.php
TimestampDatetimeWidget.php in core/lib/Drupal/Core/Datetime/Plugin/Field/FieldWidget/TimestampDatetimeWidget.php
UriWidget.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/UriWidget.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.