Typescript: Casting array to union + mapped array types not preserving type order

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms:

Mapped types key order lost

Code

type Initial = [1, 2, 3, 4]

type FindReplace<T extends any[], Find, Replace> = {
  [K in keyof T]: T[K] extends Find ? Replace : T[K]
}

// Map over array and replace '2'
type ResultArray = FindReplace<Initial, 2, 'was 2'>
// => [1, "was 2", 3, 4]

// Now accessing the array values using [number], the type
// order is lost. This matters because now when the union is later
// cast as a intersection, the types are in the wrong order
type Result = ResultArray[number]
// => 1 | 3 | 4 | "was 2

Expected behavior:

Mapping over types should preserve the key order

Actual behavior:

Mapping over types preserves the "type", but the key order is lost when performing T[keyof T] operator

Related Issues: