Added regression test

Author: staabm

tests/PHPStan/Rules/Variables/IssetRuleTest.php

@@ -492,4 +492,16 @@ public function testIssetAfterRememberedConstructor(): void
 		]);
 	}
 
+	public function testPr4374(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+
+		$this->analyse([__DIR__ . '/data/pr-4374.php'], [
+			[
+				'Offset string on array<PR4374\Foo> in isset() always exists and is not nullable.',
+				23,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/pr-4374.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace PR4374;
+
+class Foo {}
+
+final class AnnotationsMethodsClassReflectionExtension
+{
+
+	/** @var Foo[][] */
+	private array $methods = [];
+
+	public function hasMethod(string $cacheKey, string $methodName): bool
+	{
+		if (!isset($this->methods[$cacheKey][$methodName])) {
+			$method = $this->findClassReflectionWithMethod();
+			if ($method === null) {
+				return false;
+			}
+			$this->methods[$cacheKey][$methodName] = $method;
+		}
+
+		return isset($this->methods[$cacheKey][$methodName]);
+	}
+
+	private function findClassReflectionWithMethod(
+	): ?Foo
+	{
+		if (rand(0,1)) {
+			return new Foo();
+		}
+		return null;
+	}
+}