Fix crash when a partial skips a param with a constant default

[iliaal]

A partial that leaves an optional parameter unbound via a named-argument gap, where that parameter's default is a constant expression, crashes the compiler. f(a: 10, c: ?) for function f($a, $b = SOME_CONST, $c = 0) compiled the callee default into the generated forwarding call as a ZEND_AST_CONSTANT node, which zend_compile_expr_inner has no case for: ZEND_ASSERT(0) in debug, __builtin_unreachable UB in release. A plain literal default ($b = 5) is unaffected.

For a user function, this skips the unbound parameter in the forwarding call and passes the following arguments by name, so the target applies its own default at call time. That matches a normal call exactly (verified for global constants, self:: class constants, and enum cases) and materializes no value, so it is safe under opcache for defaults that cannot be stored as a literal, such as enum cases. Internal functions are unchanged: they still pass an explicit default and error when it is not introspectable.

[arnaud-lb]

Interesting one. It looks like an edge case with ZEND_AST_CONSTANT vs ZEND_AST_CONST: The compiler knows how to compile ZEND_AST_CONST, but const exprs represent const fetches as ZEND_AST_CONSTANT, which the compiler doesn't understand.

Unfortunately, passing by name doesn't work in all cases (or requires more work), for example:

function f($a, $b = K, $c = 0, ...$args) {
    return [$a, $b, $c];
}

$p = f(a: 10, c: ?, foo: 1);

Will generate:

static function ($c) use($a, $extra_named_params) {
    return \f($a, c: $c, ...$extra_named_params);
}

which is invalid due to the use of unpacking after a named param. We would need to unpack at compile time, but we may run into other issues.

Possible alternative ways to fix this:

• Support ZEND_AST_CONSTANT in zend_compile_expr_inner()
• Convert ZEND_AST_CONSTANT to ZEND_AST_CONST nodes in zp_compile_forwarding_call() (recursively)
• Remove ZEND_AST_CONSTANT to use only ZEND_AST_CONST in const exprs

I have a preference for the first one. wdyt?