[Typescript 5] infer Constraints

Since typescript 5, we are able to add constraints over infer.

Following code doesn't apply constraints, so the inferred element could be stringand number

type GetFirstStringIshElement<T> = T extends readonly [
  infer S,
  ..._:any[]
] ? S : never

const t1 = ["success", 2, 1, 4] as const
//     ^?
const t2 = [4, 54, 5] as const
//     ^?
let firstT1: GetFirstStringIshElement<typeof t1>
//   ^? success
let firstT2: GetFirstStringIshElement<typeof t2>
//   ^? 4

 

Now we want inferred element only being string

Then what we can do is:

type GetFirstStringIshElement<T> = T extends readonly [
  infer S extends string,
  ..._:any[]
] ? S : never

const t1 = ["success", 2, 1, 4] as const
//     ^?
const t2 = [4, 54, 5] as const
//     ^?
let firstT1: GetFirstStringIshElement<typeof t1>
//   ^? success
let firstT2: GetFirstStringIshElement<typeof t2>
//   ^? never