Language: circular class constants Error says Undefined constant — Zend “Cannot declare self-referencing constant” (zend_constants.c)

[PurHur]

Category

Language: · php-src-strict · class constant evaluation

Problem

Mutual / self-referencing class constants must throw Error: Cannot declare self-referencing constant self::Y (Zend detects the cycle while declaring/evaluating). VM instead surfaces Error: Undefined constant A::X when the cycle is first read.

Probed 2026-08-17 — host Zend PHP 8.2 vs php bin/vm.php.

Repro	Zend 8.2+	VM (2026-08-17)
class A { public const X = self::Y; public const Y = self::X; } then echo A::X	Error: Cannot declare self-referencing constant self::Y	Error: Undefined constant A::X

class A {
    public const X = self::Y;
    public const Y = self::X;
}
try {
    echo A::X;
} catch (Throwable $e) {
    echo $e->getMessage(), "\n";
}

php-src reference

• php/php-src Zend/zend_constants.c — self-referencing constant detection / Cannot declare self-referencing constant
• php/php-src Zend/zend_compile.c — class const AST evaluation / cycle marking

PHP implementation target

• lib/ class-constant evaluation / resolution — detect cycles and throw Zend’s self-referencing Error (do not report a one-sided “Undefined constant” when the peer name exists but is mid-evaluation)
• php-in-PHP — no new C runtime branch

Repro

./script/docker-exec.sh -- bash -lc 'php test/repro/maintainer_gap_self_referencing_class_const.php'
./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_self_referencing_class_const.php'

Done when

• Circular class consts throw Zend’s Cannot declare self-referencing constant … Error (message + that it is thrown on access/declare consistently with Zend)
• Non-circular self::OTHER class consts still resolve
• Compliance .phpt guard under test/compliance/cases/
• php-src-strict; no php-compiler-strict shortcut

[PurHur]

claim: worker-lane-B — starting this run

[PurHur]

claim: lane-B compile/link worker — starting this run

[PurHur]

merged via #31853

Verification:

• php bin/vm.php test/repro/maintainer_gap_self_referencing_class_const.php → Error:Cannot declare self-referencing constant self::Y (matches Zend)
• ./script/phpunit.sh --filter ClassConstSelfReferencingCycle31837VMTest → OK (1 test)

[PurHur]

Note: worker-lane-B also implemented this in #31854 but lost the merge race to #31853 (already merged). Repro on current master:

php bin/vm.php test/repro/maintainer_gap_self_referencing_class_const.php → Error:Cannot declare self-referencing constant self::Y