Add non-regression test for phpstan/phpstan#13623

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-13623.php

@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace Bug13623;
+
+use function PHPStan\Testing\assertType;
+
+function (array $results): void {
+	$customers = [];
+
+	foreach ($results as $row) {
+		$customers[$row['customer_id']] ??= [];
+		$customers[$row['customer_id']]['orders'] ??= [];
+		$customers[$row['customer_id']]['orders'][$row['order_id']] ??= [];
+
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['balance_forward'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['new_invoice'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['payments'] ??= 0;
+		$customers[$row['customer_id']]['orders'][$row['order_id']]['balance'] ??= $row['order_total'];
+	}
+
+	assertType("array<array{orders: array<array{}|array{balance_forward?: 0, new_invoice?: 0, payments?: 0, balance?: mixed}>}>", $customers);
+};