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To: Stephen Reay <php-lists@koalephant.com>, PHP Internals
	<internals@lists.php.net>
Thread-Topic: [PHP-DEV] Trait constants
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Date: Sat, 27 Jun 2020 19:32:20 +0000
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Subject: Re: [PHP-DEV] Trait constants
From: tysonandre775@hotmail.com (tyson andre)

> Can anyone working on internals explain why traits don=92t allow constant=
s (either technically or philosophically)?=0A=
> Moreover, what=92s the opinion(s) of the list, on adding support for this=
? Would an RFC be needed? =0A=
=0A=
I don't think there's any insurmountable obstacles, but it would be useful =
to add a lot of tests because of potential edge cases (resolution, opcache,=
 reference counting)=0A=
involved in traits and adding new places where constants can go.=0A=
=0A=
(I'd also worked on a declined RFC and implementation for changing what cou=
ld be used in a constant expression)=0A=
=0A=
```=0A=
<?php=0A=
trait T {=0A=
    // Similarly, if constants could be declared in traits,=0A=
    // it would be useful to have the same behavior of `echo ClassOrTrait::=
MY_CONST`.=0A=
    // const MY_CONST =3D self::OTHER_CONST=0A=
    // const MY_CONST_2 =3D self::CONST_DEFINED_BOTH_IN_TRAIT_AND_CLASS=0A=
    //=0A=
    // MY_CONST_2 could probably be correctly implemented if=0A=
    // subclasses copied the AST of the class constant's expression=0A=
    // instead of the evaluated value if self:: or static:: (or __CLASS__?)=
 were referenced.=0A=
=0A=
    public static function main($x =3D self::OTHER_CONST) {=0A=
        echo $x, "\n";=0A=
    }=0A=
}=0A=
// Getting reference counting correct is important to test=0A=
// strtolower() is locale-dependent, so the value isn't immutable.=0A=
define('REFERENCE_COUNTED_VALUE', strtolower('From C'));=0A=
class C {=0A=
    use T;=0A=
    const OTHER_CONST =3D REFERENCE_COUNTED_VALUE;=0A=
}=0A=
class D {=0A=
    use T;=0A=
    const OTHER_CONST =3D 'From D';=0A=
}=0A=
C::main();=0A=
try {=0A=
    T::main();=0A=
} catch (Throwable $x) {=0A=
    echo $x->getMessage() . "\n";=0A=
}=0A=
D::main();=0A=
/*=0A=
Output:=0A=
=0A=
from c=0A=
Undefined class constant 'self::OTHER_CONST'=0A=
From D=0A=
 */=0A=
```=0A=
=0A=
- Tyson=