Circularity error in intersection of object with index signature

[ahejlsberg]

TypeScript Version: 4.0.0-dev

Code

type Foo1 = { x: string } & { [x: number]: Foo1 };    // Circularity error
type Foo2 = { x: string } & { [K in number]: Foo2 };  // No error

Expected behavior:

No errors.

Actual behavior:

Error: Type alias 'Foo1' circularly references itself.

Playground Link:

https://www.typescriptlang.org/play/index.html?ssl=1&ssc=1&pln=3&pc=1#code/C4TwDgpgBAYg9nAjFAvFA3lAHgLigZ2ACcBLAOwHMoBfKAMgygG1coyBXAWwCMIiBdPPCQ0A3FAlQA9FKgBhEkQDG7ADYBDUqCh8icIgChQkWAgBMqRq0KlKNeoyYBpKOTZdeAoebESZUADk4HSI9QyA

[ahejlsberg]

Noticed this as I was writing some code. Issue is we check for empty object types in intersections, which currently causes resolution of object type members, which in turn causes unexpected circularities when the types have index signatures. We need to revise the empty object type check to not require member resolution (which really isn't necessary).

[DanielRosenwasser]

Brian Terlson (@bterlson) I think you ran into some weird code that wasn't working here. Check out the packed playground when it's ready over at #38673 (comment)

[DanielRosenwasser]

Glad this was a bug, others on the team were telling me this behavior was expected which made no sense because...well, it was a big feature in 3.7!