HttpClient should support hybrid type options

I’m submitting a…

[ ] Regression (a behavior that used to work and stopped working in a new release)
[x] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

HttpClient methods don’t support options that are typed as, e.g. { observe: 'body' | 'events' } observe has the type HttpObserve which is 'body' | 'events' | 'response', which is supposed to accept the above type. The signature overloading seems to need to repeat the signature of the final definition.

Expected behavior

HttpClient methods work with options that are typed as, e.g. { observe: 'body' | 'events' }

Minimal reproduction of the problem with instructions

function f(x: string): string;
function f(x: number): number;
// function f(x: string | number): any;
function f(x: string | number): any {
    return x;
}

interface X {
    x: number | string;
}

function x(a: X) {
    f(a.x);  // this reports an error, needs line 3 to fix (which I think is absurd)
}

Argument of type 'string | number' is not assignable to parameter of type 'number'.
  Type 'string' is not assignable to type 'number'.

What is the motivation / use case for changing the behavior?

I need more flexible typing in writing my app specific wrapper of HttpClient.

Environment

Angular version: 4.3.2

EDIT

Looks like HttpClient.request correctly repeats the final signature before the real implementation. None of the other methods (get, post, etc) do the same. Must be missed out. So my current workaround would be to use request for all the things…

[artuska]

This does work with the http:

let config = {
    params: params,
    observe: 'response'
};

let request = this.http.get(path, config);

This does not work with the httpClient:

let config = {
    params: params,
    observe: 'response'
};

let request = this.httpClient.get(path, config);

Because httpClient is stricted to observe?: "body".

Just saying.

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