Language: nullsafe operator (?->) — short-circuit + error semantics parity

[PurHur]

Category

language

Problem

Nullsafe calls (?->) are a PHP 8 language feature with specific short-circuit semantics:

• if the receiver is null, the entire chain evaluates to null without evaluating arguments or side effects
• if the receiver is non-null but not an object, Zend raises a type error consistent with normal ->

No open parity issue was found for the nullsafe operator.

php-src reference

• Zend/zend_compile.c — AST nodes and opcode emission for nullsafe chains
• Zend/zend_execute.c — runtime short-circuit behavior

Repro (failure today)

Save as repro_nullsafe.php:

<?php
class C { public function f($x) { echo "call\n"; return $x; } }

$c = null;
var_export($c?->f((function(){ echo "arg\n"; return 1; })()));
echo "\n";

$c = new C();
var_export($c?->f(2));
echo "\n";

Run:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php repro_nullsafe.php
# Zend expected:
# NULL
# call
# 2

php bin/vm.php repro_nullsafe.php
# today: parse/compile failure or wrong evaluation order
'

Scope (this repo)

• Compiler lowering for nullsafe property/method access and chaining
• VM execution: argument evaluation must be skipped when receiver is null
• Ensure correct interaction with ??, isset, and nested expressions

Done when

• repro_nullsafe.php output matches Zend on VM
• Side effects in arguments are not executed when receiver is null
• Error types/messages match Zend for non-null non-object receivers

Links

• North star: Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492

[PurHur]

claim: worker-lane-c — starting this run

[PurHur]

Merged via PR #4399.

Verified:
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php vendor/bin/phpunit test/compliance/VMTest.php --filter nullsafe --testdox'