cgen: fix markused for option generic param (fix #25501) (#25525)

Author: felipensp

vlib/v/markused/walker.v

@@ -1100,16 +1100,23 @@ pub fn (mut w Walker) call_expr(mut node ast.CallExpr) {
 		} else if node.return_type.has_flag(.result) {
 			w.used_result++
 		}
-		if stmt.params.len > 1 && stmt.generic_names.len > 0 {
+		if ((node.is_method && stmt.params.len > 1) || !node.is_method)
+			&& stmt.generic_names.len > 0 {
 			// mark concrete []T param as used
-			for concrete_type_list in w.table.fn_generic_types[node.fkey()] {
+			max_param_len := if node.is_method { stmt.params.len - 1 } else { stmt.params.len }
+			param_i := if node.is_method { 1 } else { 0 }
+			for concrete_type_list in w.table.fn_generic_types[fn_name] {
 				for k, concrete_type in concrete_type_list {
-					if k >= stmt.params.len - 1 {
+					if k >= max_param_len {
 						break
 					}
-					param_typ := stmt.params[k + 1].typ
-					if param_typ.has_flag(.generic) && w.table.type_kind(param_typ) == .array {
-						w.mark_by_type(w.table.find_or_register_array(concrete_type))
+					param_typ := stmt.params[k + param_i].typ
+					if param_typ.has_flag(.generic) {
+						if w.table.type_kind(param_typ) == .array {
+							w.mark_by_type(w.table.find_or_register_array(concrete_type))
+						} else if param_typ.has_flag(.option) {
+							w.used_option++
+						}
 					}
 				}
 			}

vlib/v/tests/options/option_generic_array_mut_test.v

@@ -0,0 +1,11 @@
+fn test_main() {
+	mut arr := []int{}
+	mut bbb := unwrap(mut arr)
+	bbb << 1
+	assert bbb.len == 1
+	assert bbb == [1]
+}
+
+fn unwrap[T](mut t ?&T) T {
+	return t or { panic('unexpected `none`') }
+}