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

[iliaal]

A partial that leaves an optional parameter unbound through a named-argument gap crashes the compiler when that parameter's default is a constant expression: f(a: 10, c: ?) against function f($a, $b = SOME_CONST, $c = 0). Const exprs represent a constant fetch as ZEND_AST_CONSTANT when it could not be evaluated at compile time, and zend_compile_expr_inner() has no case for that kind, so compiling the callee default into the generated forwarding call hits ZEND_ASSERT(0) in debug and __builtin_unreachable in release. A literal default is a ZEND_AST_ZVAL and is unaffected.

zend_compile_expr_inner() now compiles ZEND_AST_CONSTANT to ZEND_FETCH_CONSTANT, reusing the name zend_compile_const_expr_const() already resolved along with its unqualified-in-namespace flag. The constant is looked up when the partial is invoked, as in a direct call, and its value is never folded into the forwarding op_array's literals.

zp_compile_forwarding_call() also dropped the reference it takes on a constant-expression default value, which leaks on master for class-constant and enum-case defaults.

[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?

[iliaal]

Went with the first one. zend_compile_expr_inner() now compiles ZEND_AST_CONSTANT to ZEND_FETCH_CONSTANT, reusing the name zend_compile_const_expr_const() already resolved plus its unqualified-in-namespace flag, so the constant is looked up when the partial is invoked, not when it is created. Your f(a: 10, c: ?, foo: 1) case is in the test, along with the namespace fallback and a constant defined after the partial is created.

Also released the reference zp_get_param_default_value() takes on the default value. The IS_CONSTANT_AST branch was dropping it, which leaks on master for self::X and enum-case defaults.

[arnaud-lb]

Looks good to me! Thank you!