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Date: Mon, 23 Oct 2023 15:16:26 +0000
To: "php internals" <internals@lists.php.net>
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Subject: Re: [PHP-DEV] Constructor promoted property and default value
From: larry@garfieldtech.com ("Larry Garfield")

On Mon, Oct 23, 2023, at 2:18 PM, Pierre wrote:
> Hello internals,
>
> I stumbled upon this behavior, if I write this:
>
> ```php
> class Foo
> {
>  =C2=A0=C2=A0=C2=A0 public ?string $prop =3D null;
>
>  =C2=A0=C2=A0=C2=A0 public function __construct(?string $prop =3D null)
>  =C2=A0=C2=A0=C2=A0 {
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 $this->prop =3D $prop;
>  =C2=A0=C2=A0=C2=A0 }
> }
>
> class Bar extends Foo
> {
>  =C2=A0=C2=A0=C2=A0 public function __construct(
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 public ?string $bar =3D nu=
ll,
>  =C2=A0=C2=A0=C2=A0 ) {}
> }
>
> // Echoes nothing, but it works as expected.
> echo (new Bar())->prop;
> ```
>
> It works as intended, but if I replace the `Foo` class using:
>
> ```php
> class Foo
> {
>  =C2=A0=C2=A0=C2=A0 public function __construct(
>  =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 public ?string $prop =3D n=
ull,
>  =C2=A0=C2=A0=C2=A0 ) {}
> }
> ```
>
> It won't work anymore and I have the following error:
>
> ```
> PHP Warning:=C2=A0 Uncaught Error: Typed property Foo::$prop must not =
be=20
> accessed before initialization in php shell code:9
> ```
>
> If I understand it correctly:
>  =C2=A0- in the first case, default value is attached to the object pr=
operty,=20
> so if I omit its constructor, I have the default,
>  =C2=A0- in the second case, default value is attached to the construc=
tor=20
> parameter, and not to the object property, which means that in case th=
e=20
> parent constructor is not called in the `Bar` class, `$prop` remains=20
> initialized.
>
> It doesn't sound like a bug, but I think that many people would actual=
ly=20
> expect otherwise: that the constructor promoted property keep their=20
> default even when constructor is not explicitly called.
>
> Is there any good reason behind this ? Wouldn't it be best to change=20
> this behavior ? Would it be a risk for backward compatibility (my gues=
s=20
> is "not that much, probably not a all even") ?

Where this becomes a problem is readonly properties, since those are not=
 allowed to have default values.  (That would make them constants with w=
orse performance.)  A solution would need to be able to detect that the =
parent::__construct() isn't called, and then call it anyway, or at least=
 partially call it.  Unfortunately, I can think of many cases where such=
 a call would result in unexpected behavior.

It might be possible to resolve, but it's definitely not simple, and it =
could easily lead to weird behavior.

--Larry Garfield