8 files declare their use of ComplexDataDefinitionInterface

EntityDataDefinitionInterface.php in core/lib/Drupal/Core/Entity/TypedData/EntityDataDefinitionInterface.php
EntityFieldTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityFieldTest.php
EntityTypedDataDefinitionTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityTypedDataDefinitionTest.php
FieldItemBase.php in core/lib/Drupal/Core/Field/FieldItemBase.php
FieldItemDataDefinitionInterface.php in core/lib/Drupal/Core/Field/TypedData/FieldItemDataDefinitionInterface.php
FieldResolver.php in core/modules/jsonapi/src/Context/FieldResolver.php
TypedConfigTest.php in core/tests/Drupal/KernelTests/Config/TypedConfigTest.php
TypedDataDefinitionTest.php in core/tests/Drupal/KernelTests/Core/TypedData/TypedDataDefinitionTest.php

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