Index signature for tagged string has wrong keyof result

[ahejlsberg]

🕗 Version & Regression Information

This is the behavior in every version I tried.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAkgJlAvFAzsATgSwHYHMoBkUA3lAPpnACGuAXFAOSYIMC+A3AFCiRQBKEAMYBGJCSgBtANYQQ9eAF162AK4BbAEYR0UDt3DQBggExijAe3RwAPPAA0UVZu0A+Lvt4BpUchkhzAGb8QsLsUFAA9BGOEABu2gD8HtCepr6ygcEmYZHR8JxAA

💻 Code

type Id = string & { __tag: 'id '};
type Rec1 = { [key: Id]: number };
type Rec2 = Record<Id, number>;

type K1 = keyof Rec1;  // never?
type K2 = keyof Rec2;  // Id

🙁 Actual behavior

K1 is never.

🙂 Expected behavior

K1 is Id.