Language: ??= on property hooks must invoke get/set hooks (zend_property_hooks.c)

[PurHur]

Category

language

Problem

Null-coalesce assignment (??=) on a property hook must read via the get hook and write via the set hook when the backing value is unset/null — same as plain $obj->prop = … assignment in php-src.

This compiler treats ??= like a raw property write: VM emits "Attempt to read property … on null" warnings and never invokes hooks.

Distinct from #6427 (compound +=/.= operators) — ??= uses coalesce+assign opcodes (lib/Compiler.php coalesce paths).

php-src reference

• php/php-src Zend/zend_property_hooks.c — get/set dispatch on assign
• php/php-src Zend/zend_compile.c — ??= lowering

Repro

<?php
class C {
    private ?string $x = null;
    public string $y {
        get => $this->x ?? 'default';
        set => $this->x = $value;
    }
}
$c = new C();
$c->y ??= 'assigned';
echo $c->y, "\n";
// Zend: assigned

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
cat > /tmp/hook_nullcoalesce.php <<'"'"'PHPEOF'"'"'
<?php
class C {
    private ?string $x = null;
    public string $y { get => $this->x ?? "default"; set => $this->x = $value; }
}
$c = new C();
$c->y ??= "assigned";
echo $c->y, "\n";
PHPEOF
php /tmp/hook_nullcoalesce.php
php bin/vm.php /tmp/hook_nullcoalesce.php
'

Today: VM prints warnings and wrong output (not assigned).

Scope

Path	Work
lib/Compiler.php	compileCoalesce / assign paths — hook-aware dim fetch for ??= lvalues
lib/VM.php	Coalesce left branch must call get hook; right branch must call set hook
lib/JIT.php	Mirror when property has lowered hooks

Done when

• Repro prints assigned on VM without warnings
• When backing is already set, ??= is a no-op (get hook value preserved)
• test/compliance/cases/language/property_hook_nullcoalesce_assign.phpt green
• JIT/AOT pass when script contains only hook-safe opcodes

Strictness

php-src-strict

Related

• Language: compound assignment (+=, .=, …) on property hooks must use get/set hooks (zend_property_hooks.c) #6427 · Language: property hooks without set — assignment must throw Error (zend_property_hooks.c) #4821 · Language: ++/-- on property hooks must use get/set hooks (zend_property_hooks.c, pairs #6427) #6452 · Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492

[PurHur]

claim: worker-lane-C — starting this run

[PurHur]

Merged via #6493.

Fix: ??= on hooked properties now null-checks backing storage (registry `setBacking`/`getBacking`), reads via get hook on the taken branch, and writes via set hook when assigning.

Verification:

• Issue repro prints `assigned` on `bin/vm.php`
• `vendor/bin/phpunit --filter property_hook_nullcoalesce_assign test/compliance/VMTest.php` — OK (1 test)
• Related coalesce + property-hook regression (9 tests) — OK