For the motherboard has one 19-pin USB 3.0 and one 9-pin USB 2.0, e.g. MSI and Asus B450-I, Asus X470-I/X570-I, the best solution might be:
1. Connect front type-c to internal 19-pin, use 19-pin to 20-pin adapter.
2. Connect (downgrade) front type-a to internal 9-pin, use 9-pin to 19-pin...
I planned to buy ROG Strix B450-I. But now, it seems I need to pay twice more to make the front type-c work :)
Just checked ROG Strix X570-I, no 20-pin from the picture either. Hope Asus can add it before official release.