RFC: Allow static properties in enums

[TysonAndre]

These were forbiddden in the initial RFC, which was already very large/complex.
Others had suggested allowing static properties,
e.g. https://externals.io/message/112626#113037

Continue to forbid instance properties.

There are various use cases where static properties would be useful:

1. Using traits that declare static properties without getting a fatal error.
2. Static methods related to an enum that that act on immutable enum instances
   (e.g. EnvironmentEnum::current() could return ::PROD, ::STAGE, ::DEV)

The alternatives to static properties would have drawbacks,
such as shared state being more visible than intended (not private),
or not being able to fully use typed properties.

RFC: https://wiki.php.net/rfc/enum_allow_static_properties

[iluuu1994]

I'm not super convinced by the use case. Using static variables in enums would certainly not be great. Methods will behave differently in different environments even if the case is the same. This is certainly less than optimal. On the other hand, I do think that needlessly restricting the language to babysit developers is not good unless there's a technical reason to do so, which doesn't seem to be the case here.

[mvorisek]

RFC seems like it will not pass. Static variables can be emulated with a static method with a static variable.

[TysonAndre]

RFC seems like it will not pass.

Agreed.

Static variables can be emulated with a static method with a static variable.

The fact that you can do this was mentioned in the RFC https://wiki.php.net/rfc/enum_allow_static_properties#this_is_better_than_alternative_ways_that_can_be_used_to_store_shared_state

Compared to alternatives such as global variables and local static variables or static properties of placeholder classes, this is a useful option to have for the following reasons:

1. Property types can be used and enforced at runtime (and checked by type checkers).
2. Visibility is easier to enforce and read (compared to global variables or static properties declared in other classes).
3. It is easier to reset static properties in unit tests (compared to static variables).
4. This can result in more concise and easier to understand code.