Attribute parameters cannot contain BackedEnum values

[michnovka]

Description

The following code:

<?php

enum Status: string
{
    case DRAFT = 'draft';
    case PUBLISHED = 'published';
    case ARCHIVED = 'archived';
}

#[Attribute]
class ListensTo
{
    public string $event;

    public function __construct(string $event)
    {

        $this->event = $event;
    }
}

#[ListensTo(Status::DRAFT->value)]
class TestClass {}

Resulted in this output:

PHP Fatal error:  Constant expression contains invalid operations on line 23

But I expected it would work fine

PHP Version

PHP 8.1.4

Operating System

Ubuntu 20.04

[damianwadley]

->value isn't a constant expression: it requires a class instance to get the property from.

Why not use the enum properly? https://3v4l.org/BCqrS

[michnovka]

@damianwadley I tried to make the example as minimal as possible. The reason why not use the enum directly is that I need it as an array key for my implementation

<?php

enum Status: string
{
    case DRAFT = 'draft';
    case PUBLISHED = 'published';
    case ARCHIVED = 'archived';
}

#[Attribute]
class ListensTo
{
    public array $events;

    public function __construct(array $events)
    {

        $this->events = $events;
    }
}

#[ListensTo([Status::DRAFT->value => true])]
class TestClass {}

This Attribute is from a 3rd party library and I cannot change it. The key values correspond to values of enum cases I have specified elsewhere in the project. So now I have to have "magic strings" like

#[ListensTo(['draft' => true])]

And when I change Status::DRAFT, I need to change it on more places.

[iluuu1994]

There's two ways to solve this:

1. Allow -> in constant expressions
2. Allow enums (or objects) as array keys (https://wiki.php.net/rfc/object_keys_in_arrays)

Solution 1 didn't make sense before enums and before allowing new in constant expressions but I guess at this point it would be somewhat more sensible. @nikic WDYT?

[nikic]

Yeah, I think supporting -> in constant expressions makes sense.

[michnovka]

Can we target this as a bug for 8.1?

[iluuu1994]

@michnovka This is not a bug but a limitation. -> were never supported for constant expressions. New features only land in new minor/major versions of PHP.