Added regression test

Author: staabm

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -930,4 +930,9 @@ public function testBug12593(): void
 		$this->analyse([__DIR__ . '/data/bug-12593.php'], []);
 	}
 
+	public function testBug3747(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-3747.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-3747.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Bug3747;
+
+class X {
+
+	/** @var array<string,string> $x */
+	private static array $x;
+
+	public function y(): void {
+
+		self::$x = [];
+
+		$this->z();
+
+		echo self::$x['foo'];
+
+	}
+
+	private function z(): void {
+		self::$x['foo'] = 'bar';
+	}
+
+}
+
+$x = new X();
+$x->y();