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

[KiaraGrouwstra]

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 (?).

[mhegazy]

Type aliases are just that, aliases. the compiler will try to resolve the alias target once it sees it, if that causes it to loop over itself, then you will get the error. in this case a use of Flatten needs a constraint to be verified, and to do that the compiler will pull on the type, and will get into the cycle.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.