pipe expected 0 arguments about rxjs HOT 5 OPEN

const a = true;

// b is Array<number> | Array<string>
const b = a ? [1] : ['1'];

// TS 5.1
// Error: This expression is not callable.
b.filter(r => {});

const a = true;

const b: Array<number | string> = a ? [1] : ['1'];

// TS 5.1
// Now this works
b.filter(r => {});

const tmp: Observable<undefined | null> = (a ? of(undefined) : of(null));
tmp.pipe(...)

matchThingie(xx, {
  caseA: () => someObs,
  caseB: () => someOtherObs
}); // returns Observable<T> | Observable<U> so can't pipe

export interface OpFn<T, R> { (source: Observable<T>): Observable<R>; }
declare class Observable<T> {
  source: Observable<any> | undefined;
  operator: OpFn<any, T> | undefined;
  constructor(subscribe?: (this: Observable<T>, subscriber: unknown) => () => void);
  pipe(): Observable<T>;
  pipe<A>(op: OpFn<T, A>): Observable<A>;
  pipe<A, B>(op1: OpFn<T, A>, op2: OpFn<A, B>): Observable<B>;
  // all the other ones
}
declare const y: Observable<number> | Observable<string>;
y.pipe();
// sees all overloads, and somehow the one with no args always returns the first member of the union
// so if I swap number and string, pipe returns Observable<string> instead of Observable<number>

interface Unsubscribable { unsubscribe(): void; }
interface OperatorFunction<T, R> { (source: Observable<T>): Observable<R>; }
interface Observer<T> { next(value: T): void; }
declare class Observable<T> {
  pipe(): Observable<T>;
  pipe<A>(op: OperatorFunction<T, A>): Observable<A>;
  subscribe(observer: Partial<Observer<T>>): Unsubscribable;
}
declare const y: Observable<number> | Observable<string>;
y.pipe(); // only sees the first overload