Typing of toPromise is wrong

[plol]

Bug Report

Current Behavior
toPromise has the wrong type

Reproduction

const p = of<string>().toPromise() // Type is Promise<string>

Expected behavior
p will in fact resolve to undefined and should have the type Promise<string | undefined>

[cartant]

This is a fairly subtle problem.

The promise will resolve when the observable completes - resolving with the last emitted value or with undefined if a value wasn't emitted.

However, the type declarations cannot express whether or not an observable will emit a value at runtime, so it could be argued that the type for the promise should always be Promise<T | undefined>.

That would more correctly describe the behaviour, but it would be a breaking change.

[cartant]

Perhaps a solution worth considering - for v7 - would be to allow a fallback value to be passed to toPromise.

The fallback would be used if the source completes without emitting and could default to undefined.

[benlesh]

This was fixed in #5072