fix checks for phpstan/phpstan#8929

of course just checking for ArrayAccess isn't enough - it's only valid
for index assignment operations 🤦

src/Rules/Properties/ReadOnlyPropertyAssignRule.php

@@ -5,6 +5,8 @@
 use ArrayAccess;
 use PhpParser\Node;
 use PHPStan\Analyser\Scope;
+use PHPStan\Node\Expr\SetOffsetValueTypeExpr;
+use PHPStan\Node\Expr\UnsetOffsetExpr;
 use PHPStan\Node\PropertyAssignNode;
 use PHPStan\Reflection\ConstructorsHelper;
 use PHPStan\Reflection\MethodReflection;
@@ -91,7 +93,11 @@ public function processNode(Node $node, Scope $scope): array
 				continue;
 			}
 
-			if ((new ObjectType(ArrayAccess::class))->isSuperTypeOf($propertyReflection->getNativeType())->yes()) {
+			$expr = $node->getAssignedExpr();
+			if (
+				(new ObjectType(ArrayAccess::class))->isSuperTypeOf($propertyReflection->getNativeType())->yes()
+				 && (($expr instanceof SetOffsetValueTypeExpr) || ($expr instanceof UnsetOffsetExpr))
+			) {
 				continue;
 			}
 

tests/PHPStan/Rules/Properties/ReadOnlyPropertyAssignRuleTest.php

@@ -174,7 +174,12 @@ public function testBug8929(): void
 			self::markTestSkipped('Test requires PHP 8.1');
 		}
 
-		$this->analyse([__DIR__ . '/data/bug-8929.php'], []);
+		$this->analyse([__DIR__ . '/data/bug-8929.php'], [
+			[
+				'Readonly property Bug8929\Test::$cache is assigned outside of the constructor.',
+				19,
+			],
+		]);
 	}
 
 }

tests/PHPStan/Rules/Properties/data/bug-8929.php

@@ -14,6 +14,8 @@ public function __construct()
 
     public function add(object $key, mixed $value): void
     {
-        $this->cache[$key] = $value;
+        $this->cache[$key] = $value; // valid offset access
+        unset($this->cache[$key]);   // valid offset access
+        $this->cache = new \WeakMap(); // reassigning is invalid however
     }
 }