Language: ?? on property hooks must not invoke get hook when backing is initialized (zend_property_hooks.c)

[PurHur]

Category

language · php-src-strict

Problem

The null-coalescing operator ?? on a hooked property must check whether the property backing is initialized without calling the get hook (same rule as isset()). This compiler invokes the get hook before applying ??, so throwing getters emit a catchable/uncaught exception even when the backing value is present.

php-src reference

• php/php-src Zend/zend_property_hooks.c — zend_isset_property_ex, coalesce fast path
• php/php-src Zend/zend_compile.c — ZEND_AST_COALESCE on hooked properties

Repro

Save as test/repro/issue_property_hook_coalesce.php:

<?php
class C {
    public string $x {
        get { throw new Exception('get must not run for ??'); }
        set => $this->backing = $value;
    }
    private string $backing = 'a';
}
$c = new C();
var_dump($c->x ?? 'default');
echo "ok\n";

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

Runtime	Output
Zend PHP 8.4+	string(1) "a" then ok
VM (wrong)	Exception from get hook (may still print default after unwinding)

Also cover uninitialized backing: var_dump($c->x ?? 'default'); after unset($c->x) when unset is allowed — must return default without get hook.

Scope (PHP-in-PHP)

Layer	Path
VM	lib/VM.php — coalesce opcode on hooked properties; share probe with ??= (#6472)
Compiler	Lowering unchanged
JIT/AOT	Optional follow-up when hook coalesce appears in hot paths
Tests	test/compliance/cases/language/property_hook_coalesce.phpt

Done when (php-src-strict)

• Repro prints string(1) "a" + ok on VM with no get-hook invocation
• Uninitialized-backing ?? returns default without get hook
• Pairs with Language: empty() on property hooks must not invoke get hook (zend_property_hooks.c) #8901 (empty/isset family)
• ./script/ci-fast.sh --filter property_hook_coalesce green

Related

#8901 empty/isset · #6472 ??= backing check · #8870 foreach by-ref · #1492

[PurHur]

claim: worker-lane-c — starting this run

[PurHur]

Merged via #8915.

Verification (Docker):
```bash
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro/issue_property_hook_coalesce.php
./vendor/bin/phpunit --filter property_hook_coalesce
'
```
`string(1) "a"` + `ok` + unset backing → `default`; compliance green.