const doSomething = <
  B extends Bar<F>,
  F extends Foo,
>(
  FooConstructor: { new(...args : any[]): F; },
  BarConstructor: { new(...args : any[]): B; },
): B => {
  return new BarConstructor({});
}

const mything = doSomething(CustomFoo, CustomBar);

Unfortunately the type of mything is Bar<CustomFoo> whereas I require it to be CustomBar<CustomFoo>.

I would have expected TypeScript to infer the return type.