Make cast and ofType operate on wildcard types rather than Any

[ansman]

Maybe.ofType, Maybe.cast and Single.cast are declared as extensions on Maybe<Any>/Single<Any> rather than on Maybe<*>/Single<*> which prevents them from being used in some cases.

This PR fixes this and will align them with how the Observable and Flowable extensions are implemented.

This would be a binary compatible change and all existing consumers wouldn't need to change anything.

[stepango]

@ansman could you provide few examples of cases you are talking about?

[ansman]

The most obvious example that doesn't work is this:

val maybe: Maybe<*> = Maybe.just("foo")
val stringMaybe = maybe.ofType<String>()

You can only cast Maybe to Maybe<*> if you don't want unchecked warnings so it would make sense to support them.

And besides, it should be implemented like it is for Observable and Flowable which it currently isn't.

[stepango]

@ansman but this example is not correct from RxJava perspective * effectively mean Any? but potentially you can't have nulls in your stream.

val maybe: Maybe<Any> = Maybe.just("foo")
val stringMaybe = maybe.ofType<String>()

will work just fine.

[JakeWharton]

Sure but then the other extensions are needlessly wide in the types they accept.

I don't see any reason to not accept this PR for parity's sake alone.

[D4Muck]

I mean just a simple Single.just("test").cast<CharSequence>() does not work...
Type mismatch: inferred type is Single<String> but Single<Any> was expected

So I would vote for this to get merged

[thomasnield]

I have a sense that this goes beyond the scope of RxKotlin. Such extension functions can be implemented on one's own domain if they are needed.

[ansman]

I'm sorry but this feels like an extremely bad reason for closing this. This does not add, change or remove functionally but rather fix something that is, in my opinion, broken.

The built in cast function works on all types of observables, including Observable<?>. Therefore it stands to reason that these extensions should too.

[thomasnield]

Sorry I completely skimmed through this too quickly. This makes sense.

[thomasnield]

@ansman can you resolve conflict and then I will merge?

[ansman]

@thomasnield Thanks, fixed