7 files declare their use of EntityReferenceFieldItemListInterface

EntityReferenceFieldNormalizer.php in core/modules/jsonapi/src/Normalizer/EntityReferenceFieldNormalizer.php
EntityReferenceFormatterBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
FileMediaFormatterBase.php in core/modules/file/src/Plugin/Field/FieldFormatter/FileMediaFormatterBase.php
ImageFormatterBase.php in core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php
Relationship.php in core/modules/jsonapi/src/JsonApiResource/Relationship.php
ResourceIdentifier.php in core/modules/jsonapi/src/JsonApiResource/ResourceIdentifier.php
ResourceObjectNormalizer.php in core/modules/jsonapi/src/Normalizer/ResourceObjectNormalizer.php

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