Rxjs: Allow using Typescript 4.0 RC for more type info

[Zcating]

As you can see, such as forkJoin can only support full type deduction when the number of args less then 6.

Using typescript 4.0 can allow type deduction for infinity args.

[kwonoj]

Dupe of https://github.com/ReactiveX/rxjs/issues/5599

Core team tracks internal roadmap when we can use 4.0 features which'll require breaking change to support users on tsc lower than 4.0.

[P4]

@kwonoj did the team research the possibility of shipping different types depending on version?

Disclaimer: I have no idea how practical this is, but it would certainly be nice if using rxjs under 4.0 could take advantage of the new features while older TS versions got type info in its current form.

[Zcating]

@kwonoj Have you worked on TS 4.0? I would like to provide some help.

[kwonoj]

the team research the possibility of shipping different types depending on version

Core team once had discussed about this. We do use tsc to create type interfaces and tsc doesn't provide to way to create against multiple version targets, requires manual editing which we do not pursue at the moment. Moreover we do not have setup to validate / test across different interfaces per version.

Please use original issue to track progress / discuss. I'll close this as dupe.