Test case to demonstrate erroneous static analysis in PhpStorm

class-grouped-numbers.php

<?php declare(strict_types = 1);

final class Grouped_Numbers {

	/**
	 * Grouped numbers.
	 *
	 * @var array<string, int[]>
	 */
	private $grouped_numbers;

	/**
	 * Constructor.
	 *
	 * @param int[] $numbers Numbers.
	 */
	public function __construct( array $numbers ) {
		$this->grouped_numbers = $this->group_numbers( $numbers );
	}

	/**
	 * Adds a new number.
	 *
	 * @param int $number New number.
	 */
	public function add( int $number ) {
		// 👉 The use of array_values() is required or else PhpStorm's static analysis complains
		// below when passing $numbers into the group_numbers method: 
		// > Expected parameter of type 'int[]', 'int[][]' provided .
		// Interestingly, this is also bypassed by just putting parentheses around $this->grouped_numbers.
		$numbers = array_merge( ...array_values( $this->grouped_numbers ) );
		array_unshift( $numbers, $number );
		$this->grouped_numbers = $this->group_numbers( $numbers );
	}

	/**
	 * Groups numbers.
	 *
	 * @param int[] $numbers URL metrics.
	 * @return array<string, int[]> Grouped numbers.
	 */
	private function group_numbers( array $numbers ): array {
		$groups = array(
			'negative' => array(),
			'positive' => array(),
		);

		foreach ( $numbers as $number ) {
			if ( $number < 0 ) {
				$groups['negative'][] = $number;
			} else {
				$groups['positive'][] = $number;
			}
		}

		return $groups;
	}

	/**
	 * Prints the grouped numbers.
	 */
	public function print() {
		print_r( $this->grouped_numbers );
	}
}

$group = new Grouped_Numbers( array( -1, 1 ) );
$group->add( 0 );
$group->print();