TypeScript: `startWith`: multiple generics

Bug Report

Reproduction

declare const foo$: Observable<{ foo: number }>
// Expected type: { foo: number }
// Actual type: { foo: number } | { foo: number }
const bar$ = foo$.pipe(startWith({ foo: 1 }))

Although the expected/actual types are compatible, it’s slightly irritating to deal with a union type T | T instead of a consolidated type T, as it makes type inspection in VS Code much more difficult (especially when the types are large or deeply nested).

Is there a way to tell startWith to consolidate its generics if they are the same? I tried adding an overload with just one generic, but then other tests started to fail.

[cartant]

I’ve confirmed that this dtslint expectation fails:

const o = of<{ foo: number }>().pipe(startWith({ foo: 1 })); // $ExpectType Observable<{ foo: number; }>

ERROR: 5:3  expect  Expected type to be:
  Observable<{ foo: number; }>
got:
  Observable<{ foo: number; } | { foo: number; }>

And this one passes:

const v = { foo: 1 };
const o = of<typeof v>().pipe(startWith(v)); // $ExpectType Observable<{ foo: number; }>

However, I’d suggest that this is a TypeScript issue and for that reason I’d be reluctant to implement - what would be, IMO - a workaround.

I cannot see the reason for TypeScript’s not consolidating a union of identical shapes.

[cartant]

This remains an issue with RxJS 7.1 and TypeScript 4.3, but, IMO, this could still be closed as TypeScript quirk. I note that the related TypeScript issue is still open.