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Date: Mon, 3 Dec 2018 09:44:58 -0700
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Subject: Re: [RFC][Discuss] Covariant return- and contravariant parameter- types
From: levim@php.net (Levi Morrison)

On Mon, Nov 26, 2018 at 2:25 PM Levi Morrison <levim@php.net> wrote:
>
> I am happy to announce my latest RFC, [Covariant Returns and
> Contravariant Parameters][1], is open for discussion. If you do not
> recognize those terms then hopefully the introduction coupled with the
> [examples][2] will help. Here is one snippet that demonstrates
> covariant return types:
>
> <?php
> interface Repository {
>   function fetch(int $id): ?Entity;
> }
>
> class UserFactory implements Repository {
>   function fetch(int $id): ?UserEntity {
>     /* Since UserEntity is an Entity, this is now allowed */
>     /* ... implementation ... */
>   }
> }
> ?>
>
> Covariant return types and contravariant parameter types are highly
> requested features; here are a few of the bug reports:
>
>   * https://bugs.php.net/bug.php?id=71825
>   * https://bugs.php.net/bug.php?id=72442
>   * https://bugs.php.net/bug.php?id=75385
>   * https://bugs.php.net/bug.php?id=75823
>
> There is a [preliminary implementation][3] that is available. It will
> not be rebased during discussion to allow people to check out the code
> and get updates without worrying about it breaking. The code quality
> can definitely be improved, and there are still a few todo comments in
> the code.
>
> Please do not discuss the items found in [Future Scope][4] as part of
> this thread, which includes discussing types such as `mixed`,
> `scalar`, `numeric`, etc.
>
> I look forward to your feedback, and especially with your
> experimentation with the implementation.
>
> Levi Morrison
>
>   [1]: https://wiki.php.net/rfc/covariant-returns-and-contravariant-parameters
>   [2]: https://wiki.php.net/rfc/covariant-returns-and-contravariant-parameters#examples
>   [3]: https://github.com/php/php-src/compare/master...morrisonlevi:variance2
>   [4]: https://wiki.php.net/rfc/covariant-returns-and-contravariant-parameters#future_scope

Our existing engine doesn't always resolve "parent" at compile time
when possible. As part of this patch "parent" is resolved at
compile-time when possible and warnings and/or errors are emitted in
certain cases where there were none before. For instance, this would
only cause an error only if the method `C::m` were to be used:

    <?php
    class C {
      static function m(parent $m) {}
    }
    ?>

This now emits an E_DEPRECATED warning when C is compiled. In 8.0 this
should be elevated to a COMPILE_ERROR.

The current edition of the RFC fails to mention this.