As per microsoft/TypeScript#7662 the error is a breaking change to TypeScript 2.0. The fix is to block scope the variable.

Can we merge this change in? We would like to use TS 1.9 in angular2 as soon as possible.

Also, I see another error with @next, microsoft/TypeScript#8795
which will result in broken .d.ts files (but not broken compilation, which can be very confusing).

I recommend tacking on this change:

rewrite for Subject<T>:

  constructor(protected destination?: Observer<T>, protected source?: Observable<T>) {
    super();
  }

to

  constructor(destination?: Observer<T>, source?: Observable<T>) {
    super();
    this.destination = destination;
    this.source = source;
  }

so that it is clear that the arguments are optional but the field declarations are not. Or just make the field declarations in Observable optional too (but that would require all consumers to use 1.9).

Applied the feedback from @kwonoj and @rkirov that subject thing has bothered me in the past, I guess I never noted that the properties were optional... that's a strange bug for sure.

Is my understanding correct above behavior (mark field optional) will be fixed by ts compiler eventually?

I'm sure it will, but doesn't seem like much harm to make them fields to get it working today.

Those practices are widespread in our codebase not only in thise, so concerned even we apply immediate fix for this change there are still places have broken behavior. Maybe better to track compiler updates to resolve issue completely.

doesn't matter to me, I can flip it back. 😄

LGTM.

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