Use RW fetch for argument unpacking

Argument unpacking may need to create references inside the array
that is being unpacked. However, it currently can only do this
if a plain variable is unpacked, not for any nested accesses,
because the value is fetched for read. Resolve this by fetching
the operands for RW.

Author: nikic

Zend/tests/arg_unpack/nested_by_ref.phpt

@@ -0,0 +1,22 @@
+--TEST--
+By-ref unpacking of a nested access
+--FILE--
+<?php
+
+function inc(&$var) {
+    $var++;
+}
+
+$ary = [[1]];
+inc(...$ary[0]);
+var_dump($ary);
+
+?>
+--EXPECT--
+array(1) {
+  [0]=>
+  array(1) {
+    [0]=>
+    int(2)
+  }
+}

Zend/zend_compile.c

@@ -2963,7 +2963,15 @@ uint32_t zend_compile_args(zend_ast *ast, zend_function *fbc) /* {{{ */
 			uses_arg_unpack = 1;
 			fbc = NULL;
 
-			zend_compile_expr(&arg_node, arg->child[0]);
+			/* Unpacking may need to create interior references in the unpacked array,
+			 * but apart from that does not have any other reference semantics: It should
+			 * generate a notice if the variable does not exist and it should not convert
+			 * the variable itself into a reference. As such, use an RW fetch. */
+			if (zend_is_variable(arg->child[0])) {
+				zend_compile_var(&arg_node, arg->child[0], BP_VAR_RW, 0);
+			} else {
+				zend_compile_expr(&arg_node, arg->child[0]);
+			}
 			opline = zend_emit_op(NULL, ZEND_SEND_UNPACK, &arg_node, NULL);
 			opline->op2.num = arg_count;
 			opline->result.var = (uint32_t)(zend_intptr_t)ZEND_CALL_ARG(NULL, arg_count);

Zend/zend_vm_def.h

@@ -4920,7 +4920,11 @@ ZEND_VM_HANDLER(165, ZEND_SEND_UNPACK, ANY, ANY)
 	int arg_num;
 
 	SAVE_OPLINE();
-	args = GET_OP1_ZVAL_PTR_UNDEF(BP_VAR_R);
+	if (OP1_TYPE & (IS_VAR|IS_CV)) {
+		args = GET_OP1_ZVAL_PTR_PTR_UNDEF(BP_VAR_RW);
+	} else {
+		args = GET_OP1_ZVAL_PTR_UNDEF(BP_VAR_R);
+	}
 	arg_num = ZEND_CALL_NUM_ARGS(EX(call)) + 1;
 
 ZEND_VM_C_LABEL(send_again):

Zend/zend_vm_execute.h

@@ -1862,7 +1862,11 @@ static ZEND_OPCODE_HANDLER_RET ZEND_FASTCALL ZEND_SEND_UNPACK_SPEC_HANDLER(ZEND_
 	int arg_num;
 
 	SAVE_OPLINE();
-	args = get_zval_ptr_undef(opline->op1_type, opline->op1, &free_op1, BP_VAR_R);
+	if (opline->op1_type & (IS_VAR|IS_CV)) {
+		args = get_zval_ptr_ptr_undef(opline->op1_type, opline->op1, &free_op1, BP_VAR_RW);
+	} else {
+		args = get_zval_ptr_undef(opline->op1_type, opline->op1, &free_op1, BP_VAR_R);
+	}
 	arg_num = ZEND_CALL_NUM_ARGS(EX(call)) + 1;
 
 send_again: