Impose a minimum TS lib for version 7.

[benlesh]

Because v7 has interop with AsyncIterable, we need to make sure TS users have a minimum lib setting of es2018.

Issue created for discussion.

Currently, with v7, TypeScript builds targeting ES2017 and under will have issues with typings because ObservableInput<T> includes AyncIterable<T> which isn't available unless you have a lib of ES2018 or greater.

One alternative might be to do some TypeScript type meta-programming to figure out if AyncIterable was available or not.

[benlesh]

Related, it would be great if our esm build was targeting ES2018, however, I don't think we can because the esm build is what Angular uses, and they can't move past ES2015 because of Zones. I've filed an issue for that so it can be tracked: angular/angular#38782

[benlesh]

Also related: microsoft/TypeScript#40462

[cartant]

I've had a quick look at this. Adding the triple-slash reference to types.ts - as suggested here - does effect the reference in the generated .d.ts file, so manual manipulation of the .d.ts files is not required.

What we do need to do, though, is figure out just how the lib filenames work. I mean, there is no lib.esnext.asynciterable.d.ts in the TypeScript lib directory, as the type definition for AsyncIterable has not changed since the one that's in lib.es2018.asynciterable.d.ts. So does the lib="esnext.asynciterable" that's in Ryan's comment end up resolving to lib.es2018.asynciterable.d.ts ... 🤷‍♂️

[cartant]

It seems the value for the lib attribute is what would be passed to the command-line option, so lib="esnext.asynciterable" is exactly what we want.