<key> is referenced directly or indirectly in its own type annotation #22018

TypeScript Version: 2.8.0-dev.20180217

Search Terms:

• "is referenced directly or indirectly in its own type annotation"

Code

type Flatten<T> = {
  '1': Flatten<T extends Array<infer U> ? U : T>;
}[T extends number ? '1' : '1'];

(as mentioned in #21613)

Expected behavior: ok

Actual behavior:

error TS2502: ''1'' is referenced directly or indirectly in its own type annotation.

Playground Link: needs 2.8, playground is on 2.7

Related Issues: #6230

I saw similar issues on other types, though the greatest common denominator appeared to be recursion + conditional types (?).