added rule test

Author: staabm

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -924,4 +924,9 @@ public function testInternalClassesWithOverloadedOffsetAccessInvalid84(): void
 		$this->analyse([__DIR__ . '/data/internal-classes-overload-offset-access-invalid-php84.php'], []);
 	}
 
+	public function testBug12122(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-12122.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-12122.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Bug12122;
+
+function doFoo() {
+	$foo = 'mystring';
+	var_dump($foo[-1]);
+}