ReflectionProperty::getDefaultValue() and promoted properties

[damianwadley]

Copied from https://bugs.php.net/bug.php?id=81386

Currently when you declare a promoted property with a default value the ReflectionProperty::getDefaultValue() won't return that value.

I'm not sure if this behavior is expected or not but in either case something needs to be changed.

If this behavior is unexpected then it should be fixed to return the correct value.

If this behavior is expected then the documentation of ReflectionProperty::getDefaultValue() should mention this drawback along with a few lines of code how to get to the relevant ReflectionParameter and get the value there.

<?php

class X {
    public function __construct(
        public bool $y = false
    ) {
    }
}

var_export((new ReflectionProperty('X', 'y'))->getDefaultValue());

Expected result: either the value should be returned or this behavior needs to be documented.

Actual result: the default value is not returned and there is no mention of this behavior in the documentation

[damianwadley]

As has been mentioned in some places, promoted properties do not have defaults. The defaults are applied to the constructor parameters - not the properties. Thus why there is an "extra" step of looking for property information from reflected constructor data (whose ReflectionParameters do indicate whether they're for promoted properties).

[DanielEScherzer]

As noted above and elsewhere, defaults for promoted properties are only applied to the constructor parameter, not the property itself. We should say something like

Note that for promoted properties, defaults are only applied to the constructor parameter, not the property itself. Use ReflectionParameter::getDefaultValue() to retrieve the defaults for promoted properties.