Added regression test

Author: staabm

tests/PHPStan/Rules/Arrays/InvalidKeyInArrayDimFetchRuleTest.php

@@ -106,4 +106,14 @@ public function testBug13135(): void
 		]);
 	}
 
+	public function testBug12273(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-12273.php'], [
+			[
+				'Possibly invalid array key type mixed.',
+				16,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-12273.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Bug12273;
+
+function doFoo():void {
+	$map = [
+		'datetime' => \DateTime::class,
+		'stdclass' => \stdClass::class,
+	];
+
+	$settings = json_decode('{"class": "datetim"}');
+
+	\PHPStan\dumpType($map);
+	\PHPStan\dumpType($settings->class);
+
+	new ($map[$settings->class])();
+}