> Can anyone working on internals explain why traits don’t allow constants
> (either technically or philosophically)?
> Moreover, what’s the opinion(s) of the list, on adding support for this?
> Would an RFC be needed?
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)
involved in traits and adding new places where constants can go.
(I'd also worked on a declined RFC and implementation for changing what could
be used in a constant expression)
```
<?php
trait T {
// Similarly, if constants could be declared in traits,
// it would be useful to have the same behavior of `echo
ClassOrTrait::MY_CONST`.
// const MY_CONST = self::OTHER_CONST
// const MY_CONST_2 = self::CONST_DEFINED_BOTH_IN_TRAIT_AND_CLASS
//
// MY_CONST_2 could probably be correctly implemented if
// subclasses copied the AST of the class constant's expression
// instead of the evaluated value if self:: or static:: (or __CLASS__?)
were referenced.
public static function main($x = self::OTHER_CONST) {
echo $x, "\n";
}
}
// Getting reference counting correct is important to test
// strtolower() is locale-dependent, so the value isn't immutable.
define('REFERENCE_COUNTED_VALUE', strtolower('From C'));
class C {
use T;
const OTHER_CONST = REFERENCE_COUNTED_VALUE;
}
class D {
use T;
const OTHER_CONST = 'From D';
}
C::main();
try {
T::main();
} catch (Throwable $x) {
echo $x->getMessage() . "\n";
}
D::main();
/*
Output:
from c
Undefined class constant 'self::OTHER_CONST'
>From D
*/
```
- Tyson
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