Constructor property promotion for final properties

On Fri, Mar 21, 2025 at 9:30 PM Daniel Scherzer daniel.e.scherzer@gmail.com
wrote:

On Fri, Mar 21, 2025 at 9:45 AM Alexandru Pătrănescu drealecs@gmail.com
wrote:

On Fri, Mar 21, 2025 at 5:20 PM Daniel Scherzer <
daniel.e.scherzer@gmail.com> wrote:

Can you clarify if the following would result in constructor property
promotion or not:

 class Foo {
     public function __construct(
         final string $bar,
     ) { }
 }

Best regards
Tim Düsterhus

Yes, that would result in constructor property promotion. I'll need to
retarget the original PR for master, but at
https://github.com/php/php-src/pull/17861 you can see in
Zend/tests/property_hooks/final_prop_promoted_2.phpt a very similar test
case.

I see. Good catch Tim.
Daniel, you don't have this exact test case there.

Initially, in 8.0, only a visibility keyword would trigger the property
promotion logic.
Later, in 8.1, also the readonly keyword would trigger this and make the
property public, without the need to mention the visibility:
https://3v4l.org/Co0gl

Now we want the same for the final keyword, to trigger the property
promotion without having a visibility keyword,
because like readonly, it would not be applicable to a parameter, but
just to a property?

--
Alex

Sorry, I completely missed that part. Yes, that should trigger promotion,
and if it doesn't then that is a bug in my implementation. When I retarget
the PR to master I'll also add a test case for promotion from just using
final.

Hi Daniel,

When discussing the Aviz RFC, I also wrote on this topic. (
https://news-web.php.net/php.internals/123488)
The main issue being that if we ever decide to use the final modifier for a
parameter, it would not be possible.
It is already now possible to use the readonly modifier for a parameter
because of this, something I wasn't aware of until this topic.

On the readonly doing the promotion by itself, I believe it was an internal
decision (that we properly documented later), but it was not discussed on
the mailing list as far as I know.
Also, I believe there might not be an actual test that validates this
behavior.

Having said that, allowing final to trigger property promotion by itself is
not such a problematic thing, IMO. I would have preferred forcing a
visibility keyword to trigger property promotions, but the cat is out of
the bag already.
Also, allowing the final modifier for constructor parameters to be
transformed as the final modifier for promoted properties is fine.

I guess that if we ever decide to use a keyword for declared variables or
parameters to disallow changing their values, we can go with some other
keyword like const, val, etc.

--
Alex

Yes, that would result in constructor property promotion. I'll need to retarget the original PR for master, but at https://github.com/php/php-src/pull/17861 you can see in Zend/tests/property_hooks/final_prop_promoted_2.phpt a very similar test case.

To answer the original question: I'm not against this change, but as it is a syntax change, I think it does warrant an RFC, even if it's a small/easy one. That's a good way to flesh out the edge cases like that.

--Larry Garfield