This commit updates the type of the `APP_INITIALIZER` injection token to
better document the expected types of values that Angular handles. Only
Promises and Observables are awaited and other types of values are ignored,
so the type of `APP_INITIALIZER` has been updated to
`Promise<unknown> | Observable<unknown> | void` to reflect this behavior.
BREAKING CHANGE:
The type of the `APP_INITIALIZER` token has been changed to more accurately
reflect the types of return values that are handled by Angular. Previously,
each initializer callback was typed to return `any`, this is now
`Promise<unknown> | Observable<unknown> | void`. In the unlikely event that
your application uses the `Injector.get` or `TestBed.inject` API to inject
the `APP_INITIALIZER` token, you may need to update the code to account for
the stricter type.
Additionally, TypeScript may report the TS2742 error if the `APP_INITIALIZER`
token is used in an expression of which its inferred type has to be emitted
into a .d.ts file. To workaround this, an explicit type annotation is needed,
which would typically be `Provider` or `Provider[]`.
Closes#40729
PR Close#40986
ca721c2972