Type cast should try to remove readonly modifier

The compiler should try to use mutable versions of readonly tuples when couldn't cast readonly tuples to mutable arrays mainly for usability of as const. The performance impact of this would be a little because the condition when this check runs is limited.

TypeScript Version: 3.7.x-dev.20200123

Search Terms:

Code

([0] as [0]) as number[];
([0] as [0]) as readonly number[];
// As the expansion of [0] as const
([0] as readonly [0]) as number[]; // error
([0] as readonly [0]) as readonly number[];

Expected behavior:
pass
Actual behavior:

Conversion of type 'readonly [0]' to type 'number[]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
  The type 'readonly [0]' is 'readonly' and cannot be assigned to the mutable type 'number[]'.(2352)

Playground Link: http://www.typescriptlang.org/play/index.html?target=99&ts=3.8.0-dev.20200123&ssl=1&ssc=1&pln=6&pc=1#code/BQbQDAugBAhgzlcECUsEDsCuBbARgUwCcQIBuAKFEjUUlXikPxgBMB7dAGwE8os8iJCgHphUAIIIALgAt8UfAA8ADjHRwAlhyhsAZrWgMAxhzhTKSGk1YceB+hhwFiZKKIWFCbQheoNr7Fy8SA6MzIF2-M5C5EA

Related Issues:

まず、TypeScript Playground または TypeScript 3.7.x-dev.20200123 を使って readonly タプルのキャスト例を再現し、次にコンパイラーの insufficient-overlap と readonly の代入可能性に関するチェックを追跡します。readonly タプルから number[] へのキャストが、中間の unknown キャストを必要とせず期待どおりに通過し、既存の安全性チェックが維持されていれば完了です。