I was refactoring some code and suddenly some type guards in my code begin causing type errors. I've reduced my problem to the small example below. I just did npm install -g typescript@next to check that this also happens with the latest version.

After some confusion and debugging it appears that the bug happens because the class Empty has no methods. If I add a random dummy method to Empty the code compiles again.

TypeScript Version: 2.1.0-dev.20160915

Code

export type FingerTree<A> = Empty | Single<A>;

export class Empty {
  constructor() {};
}

export class Single<A> {
  constructor(
    public nested: boolean
  ) {};
}

export function test(t: FingerTree<any>): boolean {
  if (t instanceof Empty) {
    return true;
  } else {
    return t.nested;
  }
}

Expected behavior:
Code compiles without errors and t has the type Singe<any> in the else clause in test.

Actual behavior:
t has the type never in the else clause and I get` the compile error:

test.ts(17,14): error TS2339: Property 'nested' does not exist on type 'never'.