Type of reverse enum lookup is string | undefined with noUncheckedIndexedAccess enabled

Bug Report

🔎 Search Terms

• enum
• noUncheckedIndexedAccess
• undefined

🕗 Version & Regression Information

This is a new feature in Typescript 4.1.

⏯ Playground Link

Playground link with relevant code

💻 Code

enum Foo {
  A, 
  B,
  C,
}

let x: string = Foo[Foo.A];

🙁 Actual behavior

With noUncheckedIndexedAccess it gives an error because the type of Foo[Foo.A] is determined to be string | undefined.

🙂 Expected behavior

I can't see any reason why the type shouldn't be string even with noUncheckedIndexedAccess enabled.

Start with the linked TypeScript Playground, enabling noUncheckedIndexedAccess and examining the reverse enum lookup Foo[Foo.A]. Trace the type-checking behavior for this expression; done means it is accepted as a string rather than reported as string | undefined.