add regression for d5284ce

Author: rajyan

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -362,4 +362,9 @@ public function testBug3171(): void
 		$this->analyse([__DIR__ . '/data/bug-3171.php'], []);
 	}
 
+	public function testBug4747(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-4747.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-4747.php

@@ -0,0 +1,14 @@
+<?php declare(strict_types = 1);
+
+namespace Bug4747;
+
+/**
+ * @param array{a_1: array{int, string}, a_2: array{int, string}, hi: int} $r
+ * @return string
+ */
+function x(array $r, string $d) : string
+{
+	assert($d === '1' || $d === '2');
+
+	return $r['a_' . $d][1];
+}