I thought about saying (c + 'A') - 'a' to make this more clear, but I think that's actually UB with signed chars- e.g. for c='a', 'a'+'A' exceeds the range

I thought about saying (c + 'A') - 'a' to make this more clear, but I think that's actually UB with signed chars- e.g. for c='a', 'a'+'A' exceeds the range of a signed 8-bit value! Promotion should save us here, but that's a bit too yikes-y for my comfort.