Assigning property value through readonly array results in "Cannot modify readonly property"

Description

The following code:

<?php
class A {
    public function __construct(
        public readonly array $a,
    ) {}
}

class B {
    public function __construct(
        public string $b,
    ) {
    }
}

$a = new A([ new B('foo') ]);
$a->a[0]->b = 'bar';

echo $a->a[0]->b;

Resulted in this output:

PHP Fatal error:  Uncaught Error: Cannot modify readonly property A::$a in /test.php:17

But I expected this output instead:

bar

If you try to change the property of an object that is inside a readonly array, based on syntax it thinks you're change the array itself and throws a Fatal, even though that property we're actually trying to change ($a->a[0]->b) is not readonly and can be changed. If the code to change the B::$b property is refactored like this the code works as expected:

$a = new A([ new B('foo') ]);
$b = $a->a[0];
$b->b = 'bar';

PHP Version

8.2.23, 8.3.11

Operating System

No response

まず、提供された再現コードを PHP 8.2.23 と 8.3.11 で実行し、一時変数を介したリファクタリング後のアクセスと比較します。PHP インタープリターで、直接の $a->[0]->b 式に対する readonly プロパティの処理を追跡します。直接代入によって B::$b が更新され、致命的なエラーなしで bar が出力されれば完了です。