useObservable state generic is incorrectly inferred

[OliverJAsh]

const y = useObservable((input$, state$) => state$, undefined, [1]);
// Actual type: `[number] | undefined`
// Expected type: `undefined`

[OliverJAsh]

Only happens when "strictFunctionTypes": false.

Workaround: use explicit generics

const y = useObservable<undefined, [number]>((input$, state$) => state$, undefined, [1]);

PR with fix: #176