cgen: fix error, when a struct with over 8 fields, is used as a method receiver directly.

vlib/v/gen/c/struct.v

@@ -22,6 +22,10 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 	if is_amp {
 		g.out.go_back(1) // delete the `&` already generated in `prefix_expr()
 	}
+	g.write('(')
+	defer {
+		g.write(')')
+	}
 	if g.is_shared && !g.inside_opt_data && !g.is_arraymap_set {
 		mut shared_typ := node.typ.set_flag(.shared_f)
 		shared_styp = g.typ(shared_typ)

vlib/v/tests/operator_overloading_on_struct_with_too_many_fields_test.v

@@ -28,3 +28,30 @@ fn test_op() {
 	dump(a)
 	dump(b)
 }
+
+struct ManyFields {
+mut:
+	f01 int
+	f02 int
+	f03 int
+	f04 int
+	f05 int
+	f06 int
+	f07 int
+	f08 int
+	f09 int
+	f10 int
+	f11 int
+	f12 int
+}
+
+fn (mf ManyFields) inc() ManyFields {
+	mut res := mf
+	res.f01 += 1
+	return res
+}
+
+fn test_a_struct_with_many_fields_can_be_used_as_receiver_directly_without_assigning_to_an_intermediate_variable() {
+	x := ManyFields{}.inc()
+	assert x.f01 == 1
+}