Stricter type definition of Subject

[hmil]

Description:

Make type definition of Subject more strict to avoid common bugs when using Subject.

Related issue (if exists):

This issue been discussed extensively in #2852 and #5066 .

Sensitive points

As discussed in the issues linked above, it is possible to omit the argument to next when the Subject is of type Subject<void>.
When the template type is anything other than void, then an argument for next is required.

The primary case where this can be a problem is when people instantiate Subject<any> and then use subject.next() with no arguments. This will break.

The present change makes the default template type of Subject void instead of any. This means that when you write new Subject(), you get a Subject<void> (instead of Subject<any>) and are therefore able to write subject.next() or subject.next(value) if value is either any or undefined.

Other good candidates for default template type are unknown and any. We can discuss whether void is the best choice here, I am not completely sure yet.

[cartant]

This LGTM, but I would like to see a test that uses Subject<void> and calls next().

[hmil]

@cartant I added tests and some documentation to explain the breaking change and the motivation for choosing void as a default type.

[cartant]

LGTM. Thanks.

[benlesh]

The only issue I see with this is that it will break usages like subject.next(), users will be forced to pass in undefined, which isn't very ergonomic. I'm not sure I want to merge this one. We should ping the TS team on what the best solution is here.

Nevermind, this is only a breaking change for TS 2.x and lower, which we would be breaking for people in v7 anyhow.