cgen: fix variadic sumtype args passing (fix #24150) (#24207)

vlib/v/gen/c/fn.v

@@ -2522,8 +2522,10 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 					}
 				} else {
 					// passing variadic arg to another call which expects same array type
-					if args.len == 1 && args[arg_nr].typ.has_flag(.variadic)
-						&& args[arg_nr].typ == varg_type {
+					if args.len == 1
+						&& ((args[arg_nr].typ.has_flag(.variadic) && args[arg_nr].typ == varg_type)
+						|| (varg_type.has_flag(.variadic)
+						&& args[arg_nr].typ == varg_type.clear_flag(.variadic))) {
 						g.ref_or_deref_arg(args[arg_nr], arr_info.elem_type, node.language,
 							false)
 					} else {

vlib/v/tests/fns/call_args_variadic_sumtype_test.v

@@ -0,0 +1,24 @@
+type Animal = Dog | Cat
+
+struct Dog {
+	name string
+}
+
+struct Cat {
+	name string
+}
+
+fn print_names(animals ...Animal) {
+	for animal in animals {
+		assert animal.name == 'Kitty'
+	}
+}
+
+fn test_main() {
+	cat := Cat{
+		name: 'Kitty'
+	}
+	mut animals := []Animal{}
+	animals << cat
+	print_names(animals)
+}