38 files declare their use of FormElement

Button.php in core/lib/Drupal/Core/Render/Element/Button.php
Checkbox.php in core/lib/Drupal/Core/Render/Element/Checkbox.php
Checkboxes.php in core/lib/Drupal/Core/Render/Element/Checkboxes.php
Color.php in core/lib/Drupal/Core/Render/Element/Color.php
Date.php in core/lib/Drupal/Core/Render/Element/Date.php
Datelist.php in core/lib/Drupal/Core/Datetime/Element/Datelist.php
Datetime.php in core/lib/Drupal/Core/Datetime/Element/Datetime.php
DeprecatedExtendsFormElement.php in core/modules/system/tests/modules/element_info_test/src/Element/DeprecatedExtendsFormElement.php
Email.php in core/lib/Drupal/Core/Render/Element/Email.php
EntityAutocomplete.php in core/lib/Drupal/Core/Entity/Element/EntityAutocomplete.php
File.php in core/lib/Drupal/Core/Render/Element/File.php
Hidden.php in core/lib/Drupal/Core/Render/Element/Hidden.php
ImageButton.php in core/lib/Drupal/Core/Render/Element/ImageButton.php
Item.php in core/lib/Drupal/Core/Render/Element/Item.php
LanguageConfiguration.php in core/modules/language/src/Element/LanguageConfiguration.php
LanguageSelect.php in core/lib/Drupal/Core/Render/Element/LanguageSelect.php
MachineName.php in core/lib/Drupal/Core/Render/Element/MachineName.php
ManagedFile.php in core/modules/file/src/Element/ManagedFile.php
Number.php in core/lib/Drupal/Core/Render/Element/Number.php
Password.php in core/lib/Drupal/Core/Render/Element/Password.php
PasswordConfirm.php in core/lib/Drupal/Core/Render/Element/PasswordConfirm.php
PathElement.php in core/lib/Drupal/Core/Render/Element/PathElement.php
Radio.php in core/lib/Drupal/Core/Render/Element/Radio.php
Radios.php in core/lib/Drupal/Core/Render/Element/Radios.php
Range.php in core/lib/Drupal/Core/Render/Element/Range.php
Search.php in core/lib/Drupal/Core/Render/Element/Search.php
Select.php in core/lib/Drupal/Core/Render/Element/Select.php
Submit.php in core/lib/Drupal/Core/Render/Element/Submit.php
Table.php in core/lib/Drupal/Core/Render/Element/Table.php
Tableselect.php in core/lib/Drupal/Core/Render/Element/Tableselect.php
Tel.php in core/lib/Drupal/Core/Render/Element/Tel.php
Textarea.php in core/lib/Drupal/Core/Render/Element/Textarea.php
Textfield.php in core/lib/Drupal/Core/Render/Element/Textfield.php
Token.php in core/lib/Drupal/Core/Render/Element/Token.php
Url.php in core/lib/Drupal/Core/Render/Element/Url.php
Value.php in core/lib/Drupal/Core/Render/Element/Value.php
VerticalTabs.php in core/lib/Drupal/Core/Render/Element/VerticalTabs.php
Weight.php in core/lib/Drupal/Core/Render/Element/Weight.php

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