It seems like $parentType can also be of type Digia\GraphQL\Type\Definition\TypeInterface; however, parameter $parentType of Digia\GraphQL\Validation...ctsWithinSelectionSet() does only seem to accept null|Digia\GraphQL\Type\...tion\NamedTypeInterface, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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foreach ($this->findConflictsWithinSelectionSet($node, /** @scrutinizer ignore-type */ $parentType) as $conflict) {