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Date: Thu, 13 Jun 2019 18:48:23 +0200
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To: Levi Morrison <levim@php.net>
Cc: "Christoph M. Becker" <cmbecker69@gmx.de>, internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary="000000000000506b9a058b374de5"
Subject: Re: [PHP-DEV] Re: [RFC][Vote] Covariant Returns and Contravariant Parameters
From: nikita.ppv@gmail.com (Nikita Popov)

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On Wed, May 8, 2019 at 6:51 PM Levi Morrison <levim@php.net> wrote:

> On Wed, May 8, 2019 at 2:06 AM Nikita Popov <nikita.ppv@gmail.com> wrote:
> >
> > On Tue, Jan 22, 2019 at 6:59 PM Levi Morrison <levim@php.net> wrote:
> >>
> >> On Tue, Jan 15, 2019 at 1:27 PM Christoph M. Becker <cmbecker69@gmx.de>
> wrote:
> >> >
> >> > On 04.01.2019 at 20:17, Levi Morrison wrote:
> >> >
> >> > > I intend to close the vote in a day or two, unless I hear of new>
> issues from Dmitry or others.
> >> > Any news here?
> >> >
> >> > --
> >> > Christoph M. Becker
> >>
> >> I sent this a week ago to Christoph only; oops.
> >>
> >> I have not heard any news. The vote is now closed. The RFC passes 39
> >> in favor to 1 against.
> >>
> >> Special thanks to Nikita and Dmitry who have helped find issues and
> >> review the patch. It will not be merged until the implementation
> >> quality is satisfactory.
> >
> >
> > As we're moving steadily towards 7.4 feature freeze, I'd like to discuss
> what we want to do with this RFC... The current implementation doesn't work
> correctly (I've done some more work in
> https://github.com/nikic/php-src/commits/variance-7.4, but it's also
> incomplete) and I have some doubts about how we're approaching this in
> general.
> >
> > This RFC really has two parts:
> > 1. The actual variance change. This is a very straightforward change and
> there are no issues here.
> > 2. The ability to check variance across multiple consecutive class
> definitions. This allows type declarations to reference classes that are
> declared later in the same file (but within one "block" of declarations).
> >
> > The second part is technically more dicey and somewhat arbitrary when
> seen in the wider scope of how class hoisting and early binding work in
> PHP: While PHP supports declaring classes "out of order" in some very
> simple cases like this...
> >
> > class B extends A {}
> > class A {}
> >
> > ...it will not work for anything more involved than that, for example
> >
> > class C extends B {}
> > class B extends A {}
> > class A {}
> >
> > will already generate a "class not found" error.
> >
> > Now the variance RFC tackles one very specific part of this
> long-standing issue: The types referenced in parameter and return types may
> be declared later in the file (even if used variantly), but all other uses
> of the types still need to respect the declaration order.
> >
> > I think that we should be separating these two issues (variance and
> declaration order), and land the simple variance support in 7.4, while
> tackling the declaration order problem *in full* separately (in PHP 8,
> because I think we may want to make some BC breaking changes, in particular
> by making the class hoisting unconditional.)
> >
> > Thoughts on this approach?
> >
> > Nikita
>
> I fully support this approach. I will prepare a patch for simple
> variance in PHP 7.4. I intend to leave the existing test cases that
> will fail without supporting consecutive declarations, but marked as
> expected failures.
>
> I think in PHP 8 we can already benefit from the [always generate
> fatal error for incompatible method signatures RFC][1]. We might also
> be able to make some improvements with compile-time errors on invalid
> "parent::" usage (previously done for PHP 7.4 but [backed out][2]),
> which might make things a bit more straightforward (it might not --
> turns out parent is not exactly what I thought it was).
>
>   [1]: https://wiki.php.net/rfc/lsp_errors
>   [2]:
> https://github.com/php/php-src/commit/deb44d405eb27a6654ad9a57c1e5f641218b22a4


An update on this: The last part of covariance support has landed [1] a few
days ago and is part of 7.4 alpha 1. As already described, full variance is
only supported in conjunction with autoloading. When working in a single
file or with explicit includes, the requirement that classes need to be
declared before being referenced (including reference in variant type
declarations) remains.

This feature turned out to be a *lot* harder than anticipated. My initial
assumption that this would mostly "just work" if we ignore the single file
case was very wrong...

Nikita

[1]
https://github.com/php/php-src/commit/8f8fcbbd397370b407dc2552c4bd6ee4ccb0e93b

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