Different versions of RxJS not supported

[ahasall]

🐞 Bug report

Command (mark with an x)

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

Is this a regression?

No

Description

The Angular CLI ecosystem supports only one version of RxJS which is 6.4.0.
I expect different versions of RxJS to be supported because Angular itself supports a broader range of RxJS versions (^6.4.0)

🔬 Minimal Reproduction

https://github.com/ahasall/rxjs-v6.4-v6.5-bug

yarn install
yarn build

🔥 Exception or Error

If you use RxJS v6.5.2 and a library that depends on RxJS v6.4.0, you may get a TS2322 error.

For example, when working with the new Architect API, I encounter the following error:
Type 'Observable' is not assignable to type 'BuilderOutputLike' while

export declare type BuilderOutputLike = Observable<BuilderOutput> | Promise<BuilderOutput> | BuilderOutput;

🌍 Your Environment

@angular-devkit/architect          0.800.0
@angular-devkit/build-angular      0.800.0
@angular-devkit/build-ng-packagr   0.800.0
@angular-devkit/build-optimizer    0.800.0
@angular-devkit/build-webpack      0.800.0
@angular-devkit/core               8.0.0
@angular-devkit/schematics         8.0.0
@ngtools/json-schema               1.1.0
@ngtools/webpack                   8.0.0
@schematics/angular                8.0.0
@schematics/update                 0.800.0
ng-packagr                         5.2.0
rxjs                               6.5.2
typescript                         3.5.1
webpack                            4.30.0

Anything else relevant?
A workaround for people encountering this issue is to downgrade their version of RxJS to 6.4.0

[alan-agius4]

Related to ReactiveX/rxjs#4818

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