mapped type with remapped keys unexpectedly widens type

🔎 Search Terms

keyof, string, record

🕗 Version & Regression Information

• This changed between versions 4.4.4 and 4.5.5

⏯ Playground Link

No response

💻 Code

type R = { [K in keyof Record<string,unknown> as K]: unknown; };
//   ^?
type K = keyof R;
//   ^?
let s:string = "a" as K

Workbench Repro

🙁 Actual behavior

keyof R is string | number.

🙂 Expected behavior

keyof R is string.

Additional information about the issue

No response