Using 'Key in keyof ArrayType' always returns an Array regardles of the value manipulation applied

TypeScript Version: 4.0.0-beta (same behaviour in version 3.5.1 v3.5.1-Play Ground)

Search Terms:
"key in keyof Array", "keyof Array"

Expected behavior:
using key in ArrayType has the same behaviour as on any object (returning an array type only if the relevant properties are unchanged)

Actual behavior:
using key in ArrayType performs actions on touple entries but the returned type allways includes all default Array properties

Extention note:
Not the point of this issue and i know there are other issues around this topic. it's just a note,
but how about possibly adding tupple functionality for ArrayLike-, Map- and Set-types? They have all the basic qualities of a tupple, and it would be awesome to use them in a similar fashion (especially inference of Map.prototype.get/set and [Symbol.iterator] inference of tupples)

Related Issues:
#39726
#34780
type-fest/issues/119 (external library, but about this this behaviour and includes adHoc fix)

Code

type Arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
type Arr2 = (0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9)[]
type WhoopMyArray_Faulty<Base extends ArrayLike<any>, Condition> = {  
		[Key in keyof Base]:
			Base[Key] extends Condition
				? 'Whoop it'
				: Base[Key] extends Base[number] ? Base[Key] : never
	}

// action is performed on array elements but return type is still a proper array
type WhoopedArray = WhoopMyArray_Faulty<Arr, 7 | 5 | 2>
type WhoopedArrayKeys = WhoopMyArray_Faulty<Arr, 7 | 5 | 2>[keyof Arr]

// related side note, but not the main pont
// no action performed. in line with 1st example, expected: (0 | 1 | 'Whoop it' | 3 | 4 | 'Whoop it' | 6 | 'Whoop it' | 8 | 9)[]
type WhoopedArray2 = WhoopMyArray_Faulty<Arr2, 7 | 5 | 2>
type WhoopedArray2Keys = WhoopMyArray_Faulty<Arr, 7 | 5 | 2>[keyof Arr]

infered types:

type WhoopedArray = [0, 1, "Whoop it", 3, 4, "Whoop it", 6, "Whoop it", 8, 9]

type WhoopedArrayKeys = 0 | "Whoop it" | 1 | 3 | 4 | 6 | 8 | 9 | 10 | (() => string) | (() => string) 
  | (() => 0 | "Whoop it" | 1 | 3 | 4 | 6 | 8 | 9 | undefined) | ((...items: (0 | "Whoop it" | 1 | 3 | 4 | 6 | 8 | 9)[]) => number)
  | {    ...;    } | ... 22 more ... | (() => IterableIterator<...>)

type WhoopedArray2 = (0 | 7 | 5 | 2 | 1 | 3 | 4 | 6 | 8 | 9)[]

type WhoopedArray2Keys = 0 | "Whoop it" | 1 | 3 | 4 | 6 | 8 | 9 | 10 | (() => string) | (() => string)
  | (() => 0 | "Whoop it" | 1 | 3 | 4 | 6 | 8 | 9 | undefined) | ((...items: (0 | "Whoop it" | 1 | 3 | 4 | 6 | 8 | 9)[]) => number)
| {     ...;    } | ... 22 more ... | (() => IterableIterator<...>)

"use strict";

{
  "compilerOptions": {
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "useDefineForClassFields": true,
    "alwaysStrict": true,
    "allowUnusedLabels": true,
    "noEmitHelpers": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "preserveConstEnums": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "moduleResolution": 2,
    "target": "Latest",
    "module": "ESNext"
  }
}

Playground Link: Provided

Inizia dal link fornito a TypeScript Playground e dagli esempi di tipi mappati che usano keyof Array e i tipi tupla. Confronta i tipi inferiti con il comportamento atteso ed effettivo indicato, quindi esamina le issue correlate #39726 e #34780. Il lavoro è completato quando il tipo mappato conserva le trasformazioni previste degli elementi tupla o Array senza includere inaspettatamente membri di Array invariati.