Language: typed trait constants (PHP 8.3) — const type enforcement when importing traits (Zend/zend_compile.c)

[PurHur]

Category

language

Problem

PHP 8.3 adds typed class constants. Typed constants inside traits must behave like Zend when imported into a class:

• The constant must carry its declared type.
• The value must be validated against the declared type at compile time (or at least fail with the same fatal/error behavior as Zend).
• If a class overrides a trait constant, type compatibility rules must match Zend.

This repo already tracks typed interface constants (#5980) and interface constant inheritance compatibility (#5982). Traits are a distinct path (trait import/flattening) and need separate parity coverage.

php-src reference

• Zend/zend_compile.c — typed constant declaration and validation
• Zend/zend_inheritance.c — trait constant import and conflict resolution

Repro

Save as test/repro-maintainer/typed_trait_constants.php:

<?php
trait TGood {
    public const int X = 1;
}

final class C {
    use TGood;
}

echo C::X, "\n";

Also validate error parity (value/type mismatch) with test/repro-maintainer/typed_trait_constants_mismatch.php:

<?php
trait TBad {
    public const int X = '1';
}

final class C {
    use TBad;
}

Run:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/typed_trait_constants.php
php bin/vm.php test/repro-maintainer/typed_trait_constants.php

php test/repro-maintainer/typed_trait_constants_mismatch.php
php bin/vm.php test/repro-maintainer/typed_trait_constants_mismatch.php
'

Expected (Zend):

• First script prints 1.
• Second script fails with a compile-time fatal/error indicating the constant value is not of type int.

Observed (this compiler today):

• Either parse/compile failure for typed constants in traits, or missing type enforcement, or incorrect constant metadata.

Scope (this repo)

• lib/Compiler.php — lowering typed const declarations inside traits
• lib/VM/ — trait constant import into class entry, preserving declared type metadata
• lib/VM/Constant* / metadata structures — ensure typed const type is carried through
• Tests:
  • test/compliance/cases/language/typed_trait_constants.phpt
  • test/compliance/cases/language/typed_trait_constants_mismatch.phpt

Done when

• Typed constants in traits compile and are accessible via C::X on VM
• Mismatch script fails with the same error class and message shape as Zend
• Trait + class override compatibility matches Zend (add a third PHPT if needed)
• ./script/ci-fast.sh --filter typed_trait_constants is green

Labels

php-src-strict (default) — error shapes and type enforcement must match Zend.

Links

• Related: Language: typed interface constants (PHP 8.3, zend_compile.c) #5980, Language: interface typed constant inheritance — child const type must be compatible (PHP 8.3, zend_compile.c) #5982
• North star: Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492

[PurHur]

claim: worker-lane-a — starting this run

[PurHur]

Merged via #6142

Root cause: php-cfg-typed-class-const.patch was not reliably applied before optional php-cfg patch failures, so trait/class typed constants lacked declaredType on CFG nodes and compile-time verifyClassConstCompileTimeType() never ran.

Fix: Early apply_php_cfg_typed_class_const_overlay() (Const_.php overlay + Parser.php anchor) with critical-marker verification.

Verification (Docker):

php bin/vm.php test/repro-maintainer/typed_trait_constants.php          # prints 1
php bin/vm.php test/repro-maintainer/typed_trait_constants_mismatch.php # exit 255, CompileError
./vendor/bin/phpunit --filter typed_trait_constants test/compliance/VMTest.php  # OK (2)
./vendor/bin/phpunit test/unit/TraitTypedConstCompileTest.php                 # OK (4)