The expression return $this->defaultValue could return the type Digia\GraphQL\Util\SerializationInterface which is incompatible with the type-hinted return null|Digia\GraphQL\Langu...Node\ValueNodeInterface. Consider adding an additional type-check to rule them out.
The method toArray() does not exist on Digia\GraphQL\Language\Node\ValueNodeInterface. Since it exists in all sub-types, consider adding an abstract or default implementation to Digia\GraphQL\Language\Node\ValueNodeInterface.
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation