Add non-regression test for bug 5271

Closes phpstan/phpstan#5271

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-5271.php

@@ -0,0 +1,40 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug5271;
+
+use function PHPStan\Testing\assertNativeType;
+use function PHPStan\Testing\assertType;
+
+class BannerPatternLayer
+{
+
+}
+
+class Banner
+{
+
+	/** @var list<BannerPatternLayer> */
+	private array $patterns = [];
+
+	public function test(): void
+	{
+		$this->patterns = array_filter($this->patterns, fn($v) => $v instanceof BannerPatternLayer);
+	}
+
+	public function testClosure(): void
+	{
+		$this->patterns = array_filter($this->patterns, function ($v) {
+			assertType('Bug5271\BannerPatternLayer', $v);
+			assertNativeType('mixed', $v);
+			return $v instanceof BannerPatternLayer;
+		});
+	}
+
+	public function testArrow(): void
+	{
+		array_filter($this->patterns, fn($v) => assertNativeType('mixed', $v));
+	}
+
+}

tests/PHPStan/Rules/Classes/ImpossibleInstanceOfRuleTest.php

@@ -596,4 +596,10 @@ public function testBug13975(string $file): void
 		$this->analyse([$file], []);
 	}
 
+	public function testBug5271(): void
+	{
+		$this->treatPhpDocTypesAsCertain = false;
+		$this->analyse([__DIR__ . '/../../Analyser/nsrt/bug-5271.php'], []);
+	}
+
 }