// ConditionalTuple could also return just [] in the false branch, issue still repros
type ConditionalTuple<T> = [T] extends [undefined] ? [] : [T];

// works fine!
declare var f1: (...args: any[]) => void;
declare var f2: <T>(...args: ConditionalTuple<T>) => void;
declare var f3: (...args: ConditionalTuple<number>) => void;
f2 = f1;
f3 = f1;

// but if we move the signatures above into a callback parameter with that signature,
// assignability breaks - the types of cb are incompatible.
declare var cb1: (cb: (...args: any[]) => void) => void;
declare var cb2: <T>(cb: (...args: ConditionalTuple<T>) => void) => void;
declare var cb3: (cb: (...args: ConditionalTuple<number>) => void) => void;

cb2 = cb1; // can't assign to cb2
cb3 = cb1; // this works though