Detect mismatch between readonly/non-readonly class parent

src/Rules/Classes/ExistingClassInClassExtendsRule.php

@@ -81,6 +81,24 @@ public function processNode(Node $node, Scope $scope): array
 					$reflection->getDisplayName(),
 				))->build();
 			}
+
+			if ($reflection->isClass()) {
+				if ($node->isReadonly()) {
+					if (!$reflection->isReadOnly()) {
+						$messages[] = RuleErrorBuilder::message(sprintf(
+							'%s extends non-readonly class %s.',
+							$currentClassName !== null ? sprintf('Readonly class %s', $currentClassName) : 'Anonymous readonly class',
+							$reflection->getDisplayName(),
+						))->nonIgnorable()->build();
+					}
+				} elseif ($reflection->isReadOnly()) {
+					$messages[] = RuleErrorBuilder::message(sprintf(
+						'%s extends readonly class %s.',
+						$currentClassName !== null ? sprintf('Non-readonly class %s', $currentClassName) : 'Anonymous non-readonly class',
+						$reflection->getDisplayName(),
+					))->nonIgnorable()->build();
+				}
+			}
 		}
 
 		return $messages;

tests/PHPStan/Rules/Classes/ExistingClassInClassExtendsRuleTest.php

@@ -119,4 +119,30 @@ public function testPhpstanInternalClass(): void
 		]);
 	}
 
+	public function testReadonly(): void
+	{
+		if (PHP_VERSION_ID < 80200) {
+			$this->markTestSkipped('This test needs PHP 8.2');
+		}
+
+		$this->analyse([__DIR__ . '/data/extends-readonly-class.php'], [
+			[
+				'Readonly class ExtendsReadOnlyClass\Foo extends non-readonly class ExtendsReadOnlyClass\Nonreadonly.',
+				25,
+			],
+			[
+				'Non-readonly class ExtendsReadOnlyClass\Bar extends readonly class ExtendsReadOnlyClass\ReadonlyClass.',
+				30,
+			],
+			[
+				'Anonymous non-readonly class extends readonly class ExtendsReadOnlyClass\ReadonlyClass.',
+				35,
+			],
+			[
+				'Anonymous readonly class extends non-readonly class ExtendsReadOnlyClass\Nonreadonly.',
+				39,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Classes/data/extends-readonly-class.php

@@ -0,0 +1,41 @@
+<?php // lint >= 8.2
+
+namespace ExtendsReadOnlyClass;
+
+class Nonreadonly
+{
+
+}
+
+readonly class ReadonlyClass
+{
+
+}
+
+class Lorem extends Nonreadonly // ok
+{
+
+}
+
+readonly class Ipsum extends ReadonlyClass // ok
+{
+
+}
+
+readonly class Foo extends Nonreadonly // not ok
+{
+
+}
+
+class Bar extends ReadonlyClass // not ok
+{
+
+}
+
+new class extends ReadonlyClass { // not ok
+
+};
+
+new readonly class extends Nonreadonly { // not ok
+
+};