How to constrain generic to a union?

Question

Does anybody know how to express: "constrain the generic so it must be a union with a certain type (in this case: null)"?

type Test<value_T extends Enforce_null_union>

Test<number | null> // Valid
Test<string | null> // Valid
Test<number> // Invalid, must be a union containing `null`
Test<number> // Invalid, must be a union containing `null`

Answer 1

With a similar idea than Nadia's answer, this should also work (playground):

type Test<T extends (null extends T ? unknown : never)> = T

We can see a pattern here: T extends S -> T extends (validation ? S : never)

With S = unknown if we don't want any particular constraint on T (other that meeting the validation).