Fix oveerriding __debugInfo with covariant return type

Author: staabm

src/Reflection/Php/PhpMethodFromParserNodeReflection.php

@@ -83,7 +83,9 @@ public function __construct(
 		}
 
 		if ($name === '__debuginfo') {
-			$realReturnType = TypeCombinator::addNull(new ArrayType(new MixedType(true), new MixedType(true)));
+			$realReturnType = TypeCombinator::intersect(TypeCombinator::addNull(
+				new ArrayType(new MixedType(true), new MixedType(true)),
+			), $realReturnType);
 		}
 
 		if ($name === '__unserialize') {

tests/PHPStan/Rules/Methods/OverridingMethodRuleTest.php

@@ -556,4 +556,10 @@ public function testBug6104(): void
 		$this->analyse([__DIR__ . '/data/bug-6104.php'], []);
 	}
 
+	public function testBug9391(): void
+	{
+		$this->phpVersionId = PHP_VERSION_ID;
+		$this->analyse([__DIR__ . '/data/bug-9391.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Methods/data/bug-9391.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Bug9391;
+
+class A
+{
+	private $a;
+
+	/**
+	 * @return array<string, mixed>
+	 */
+	public function __debugInfo(): array
+	{
+		return [
+			'a' => 1,
+		];
+	}
+
+	/**
+	 * @return array<int, string>
+	 */
+	public function __serialize(): array
+	{
+		return [
+			'a' => $this->a
+		];
+	}
+}
+
+class B extends A
+{
+	private $b;
+
+	public function __debugInfo(): array
+	{
+		return [
+			'b' => 2,
+		];
+	}
+
+	public function __serialize(): array
+	{
+		return [
+			'b' => $this->b
+		];
+	}
+
+}