added regression test

Author: staabm

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -1113,4 +1113,12 @@ public function testBug12805(): void
 		$this->analyse([__DIR__ . '/data/bug-12805.php'], []);
 	}
 
+	public function testBug6209(): void
+	{
+		$this->checkExplicitMixed = true;
+		$this->checkImplicitMixed = false;
+
+		$this->analyse([__DIR__ . '/data/bug-6209.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Arrays/data/bug-6209.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Bug6209;
+
+/**
+ * @param mixed[] $values
+ * @return mixed[]
+ */
+function apply(array $values): array
+{
+	if (!array_key_exists('key', $values)) {
+		$values['key'][] = 'any';
+	}
+
+	return $values;
+}