Excessive stack depth comparing types 'Snapshot<T>' and 'DataSnapshot'. [2321]

[rtm]

TypeScript Version: 2.1.5

Code

interface DataSnapshot {
  child(path: string): DataSnapshot;
}

interface Snapshot<T> extends DataSnapshot {
  child<U extends keyof T>(path: U): Snapshot<T[U]>;
}

Expected behavior:

Should compile.

Actual behavior:

Gives error:

Excessive stack depth comparing types 'Snapshot' and 'DataSnapshot'. [2321]

If the definition of DataSnapshot#child is changed to child(path:string): any; , then no error occurs.

[RyanCavanaugh]

@ahejlsberg thoughts?