Omitted `= null` optional arguments behave as non-null at runtime

[nahime0]

Summary

Omitted optional arguments with a default value of null are not materialized as null at runtime. Branches that test $arg !== null behave as if the argument were populated even when the caller omitted it.

Minimal repro

<?php
function f($v = null) {
    echo $v !== null ? 'x' : 'n';
}

f();

Steps to reproduce

1. Save the file above as main.php.
2. Run php main.php.
3. Run cargo run -- main.php && ./main.

Expected behavior

PHP prints:

n

Actual behavior

Elephc prints:

x

A closure variant shows the same underlying bug:

<?php
$f = function($v = null) {
    echo $v !== null ? 'x' : 'n';
};
$f();

PHP prints n, Elephc prints x.

Notes

This looks like a call-lowering/runtime argument materialization bug rather than a closure-specific issue. The same mismatch appears in regular functions and closures.

Environment

• elephc version: 0.23.7
• Host OS: macOS / Darwin arm64
• PHP reference: PHP 8.4.19 (cli)