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Message-ID: <5be74be4-972f-4eae-ae46-6d92587ccc7f@processus.org>
Date: Mon, 23 Oct 2023 16:18:58 +0200
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Subject: Constructor promoted property and default value
From: pierre-php@processus.org (Pierre)

Hello internals,

I stumbled upon this behavior, if I write this:

```php
class Foo
{
     public ?string $prop = null;

     public function __construct(?string $prop = null)
     {
         $this->prop = $prop;
     }
}

class Bar extends Foo
{
     public function __construct(
         public ?string $bar = null,
     ) {}
}

// 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
{
     public function __construct(
         public ?string $prop = null,
     ) {}
}
```

It won't work anymore and I have the following error:

```
PHP Warning:  Uncaught Error: Typed property Foo::$prop must not be 
accessed before initialization in php shell code:9
```

If I understand it correctly:
  - in the first case, default value is attached to the object property, 
so if I omit its constructor, I have the default,
  - in the second case, default value is attached to the constructor 
parameter, and not to the object property, which means that in case the 
parent constructor is not called in the `Bar` class, `$prop` remains 
initialized.

It doesn't sound like a bug, but I think that many people would actually 
expect otherwise: that the constructor promoted property keep their 
default even when constructor is not explicitly called.

Is there any good reason behind this ? Wouldn't it be best to change 
this behavior ? Would it be a risk for backward compatibility (my guess 
is "not that much, probably not a all even") ?

Regards,

--
Pierre