Readonly properties mutable when assigning reference to them

[ondrejmirtes]

Description

Readonly properties should not be mutable with any means, but the following code surprisingly works (https://3v4l.org/qiDTk):

<?php

class Foo
{
	public readonly int $bar;
	public function set(int &$i)
	{
		$this->bar = &$i;
	}
}

$i = 1;
$foo = new Foo();
$foo->set($i);
$i = 20;
var_dump($foo->bar); // 20

Additionally, without the & in int &$i, the property value stays 1 but does not report any error: https://3v4l.org/NMDAp

PHP Version

PHP 8.1

Operating System

No response

[pthreat]

However one must note that the following example works as intended:

<?php // lint >= 8.1

namespace ReadonlyPropertyPassedByRef;

class Foo
{

	public readonly int $bar;
	
	public function set(int &$i)
	{
		$this->bar = &$i;
	}

}

$i = 1;
$z = 25;
$foo = new Foo();
$foo->set($i);
$i = &$z;
var_dump($foo->bar);

The reference of the variable has been set as read only, not it's value.

[cmb69]

Not sure whether this is a bug; it might fall in the "readonly properties do not preclude interior mutability" category.

[iluuu1994]

/cc @nikic

The engine does prevent creating references from readonly properties.
https://3v4l.org/cRgNM#v8.1.1

Maybe we could do the same in the other direction.
https://3v4l.org/M1n37#v8.1.1

class Foo {
    public readonly int $bar;
    public function __construct(int &$bar) {
        // Error right here, can't assign reference
        $this->bar = &$bar;
    }
}

$i = 42;
$foo = new Foo($i);
$i++;
var_dump($foo->bar);

[ondrejmirtes]

@cmb69 I’d consider it a bug because otherwise this sentence from the RFC isn’t really true:

Reading a readonly property will always return the same value, no matter what code runs in between

[nikic]

Yeah, this should not be allowed.

[nikic]

In zend_std_get_property_ptr_ptr(), we currently allow acquiring a property pointer if the current value is undef and the scope has initialization access. This then allows zend_assign_to_property_reference() to assign a reference to the property.

[nikic]

Always falling back to read_property for readonly props means that $obj->prop[] = 1 no longer works as an initializing assignment (it will generate a "must not be accessed before initialization" error). That's probably not a big loss, as the sane way to write that would be $obj->prop = [1] (you can only do the append once). The error message for the $obj->prop =& $ref case is also not great (same "accessed before initialization" error).