Fatal error: class and trait define the same constant in the composition of child class in case constant in trait is equal to constant in child class

[MaXal]

Description

The following code:

<?php
trait T {
    public const a = C2::b;
}
class C1 {
    use T;
    const b = 1;
}
class C2 extends C1  {
    use T;
    const b = 2;
}
echo C1::a;
echo C2::a;

Resulted in this output:

Fatal error: C1 and T define the same constant (a) in the composition of C2. However, the definition differs and is considered incompatible. Class was composed in /in/riAnX on line 9

But I expected this output instead:

22

It's not clear to me why this code is incorrect, but at the same time, the following works:

<?php
trait T {
    public const a = self::b;
}
class C1 {
    use T;
    const b = 1;
}
class C2 extends C1  {
    use T;
    const b = 2;
}
echo C1::a;
echo C2::a;

and outputs

11

PHP Version

8.2

Operating System

No response

[sj-i]

Hmm, I'm not sure what to do here. Surely it's buggy, but I doubt it's easily fixable.

This isn't only for trait constants. For trait properties, this behaves the same too since PHP 5.4.
https://3v4l.org/htBnW

Early bindings of classes using traits are not supported.
So, in this case, until the compiler executor reaches the definition (DECLARE_CLASS) of C2, C2::b might be a reference to an undefined class constant that resolves to null.

For self::b, it's resolved at runtime, so doesn't cause any problems.

Edit:
No, I was wrong. A reference to an undefined class constant should trigger error. I'll look into it more.

[cmb69]

The relevant point is already shown by the warning, which is emitted by this script:

Warning: Uncaught Error: Class "C2" not found in gh9272.php:4

During execution, DECLARE_CLASS string("c1") fails to resolve the AST expression of the constant, because C2 can't be found (because this is late bound, too), and from there is goes south.

Surely it's buggy, but I doubt it's easily fixable.

I agree. And the given code appears to be quite an edge-case. I'd say we should just document the limitation, although I wouldn't know how to do this without requiring the knowledge of https://www.npopov.com/2021/10/20/Early-binding-in-PHP.html (which is not even roughly covered in the PHP manual yet).