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Language: compile-time inheritance variance — parameter/return LSP vs Zend (zend_inheritance.c parity) #3323
Description
Category
language
Problem
Zend validates return type covariance and parameter type contravariance when a child class overrides a parent method or implements an interface. This compiler accepts incompatible signatures silently, allowing calls that would be compile-time errors on php-src.
php-src reference
Zend/zend_inheritance.c—zend_check_inheritance,zend_verify_type_compatibilityZend/zend_type.c—zend_type_compare/ assignability for inheritanceZend/zend_compile.c— override validation at class compile time
Repro (today)
Contravariant parameter violation (should not compile)
<?php
interface I { public function f(B $x): void; }
class A {}
class B extends A {}
class C implements I { public function f(A $x): void {} }./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php bin/vm.php repro.php'Zend: compile error (Declaration of C::f(A $x) must be compatible with I::f(B $x)).
This compiler: prints ok (no error).
Covariant return violation
<?php
class Base { public function create(): self { return $this; } }
class Child extends Base { public function create(): Child { return $this; } }Document Zend vs this compiler before/after fix (covariant self/static child returns are allowed; incompatible narrowing is not).
Scope (this repo)
lib/Compiler.php— after method merge, call type-compatibility check against parent/interface slot- Reuse
lib/Type.php/ php-types assignability where possible test/compliance/cases/language/inheritance_variance_*.phpt- JIT/AOT: inherit compile-time rejection (no separate runtime-only hack)
Done when
- Contravariant repro fails at compile time with Zend-shaped error
- Covariant return narrowing cases covered by compliance tests
-
./script/ci-fast.sh --filter inheritance_variancegreen
Verification
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter inheritance_variance'Links
- Compiler: Class inheritance (extends) and interfaces #101 / Language: Traits, abstract classes, and interfaces #144 inheritance · Language: Typed properties, union types, and readonly (PHP 8+) #169 typed properties (separate: runtime assign checks)
- Self-host Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492
Activity
PurHur commentedon May 29, 2026
claim: cursor-agent-lane-c — starting this run
PurHur commentedon May 29, 2026
Lane C continuation — PR #3332 verified after merge with
master.Verification (Docker)
Both green (4 unit + 2 compliance).
Zend parity spot-check
I::f(B $x)/C::f(A $x)): Zend allows; compiler allows.I::f(A $x)/C::f(B $x)): Zend fatal Declaration of … must be compatible; compilerCompileErrorwith same message shape.Done-when checklist on the issue is satisfied on branch
agent/issue-3323-inheritance-variance— ready for review/merge.PurHur commentedon May 30, 2026
claim: cursor-agent-lane-c — continuing PR #3332 this run
test(compliance): add inheritance variance compile-time failure PHPTs (…
PurHur commentedon May 30, 2026
Verification (cursor-agent-lane-c, this run):
origin/masterinto PR Language: compile-time inheritance variance (LSP vs Zend) #3332.inheritance_variance_param_narrow_fail.phpt,inheritance_variance_return_wide_fail.phpt.php-src ref:
Zend/zend_inheritance.c(zend_verify_type_compatibility),Zend/zend_type.c.Language: compile-time inheritance variance (LSP vs Zend) (#3332)