It seems like $this->key() can also be of type double and string and true; however, parameter $index of Ktomk\Pipelines\File\Info\StepInfo::__construct() does only seem to accept integer, 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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return new StepInfo(parent::current(), /** @scrutinizer ignore-type */ $this->key());