I think this is a slightly different problem. The absence of an optional field, if that's a legal state, is meaningful every time you use the type, so you encode it on the field: `phone: ValidPhoneNumber | null`. When it's not null you're still guaranteed a valid phone number. When it is null, that's a legal state you have to handle and which is domain logic, not validation you forgot to do.<p>The combinatorial explosion you're picturing only shows up if you make a separate type per combination of present fields, but you don't need to. An independent optional field stays one `T | null`. You only reach for distinct types when fields are correlated and present together because they represent a state, and then it's a discriminated union on a status field, which is N states, not 2^N.