typescripttypescript-genericsunion-types

How to constrain generic to a union?

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`
Solution

  • 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 than
    meeting the validation).