interface InstanceToCreateDto { }

interface InstanceToUpdateDto {
  id: string;
}

interface FooBarCreateDto extends InstanceToCreateDto {
  plop: string;
  baz?: string | undefined;
}

interface FooBarDto extends FooBarCreateDto, InstanceToUpdateDto {
  baz: string;
}

interface PersonCreateDto extends InstanceToCreateDto {
  name: string;
}

interface PersonDto extends PersonCreateDto, InstanceToUpdateDto { }

type InstanceToCreate = FooBarCreateDto | PersonCreateDto;

type InstanceToUpdate = FooBarDto | PersonDto;

function instanceExists(instance: InstanceToCreateDto | InstanceToUpdateDto): instance is InstanceToUpdateDto {
  return !!(instance as InstanceToUpdateDto).id;
}

function createOrUpdateInstance(instance: InstanceToCreate | InstanceToUpdate) {
  if (instanceExists(instance)) {
    // update InstanceToUpdate
  }
  // create InstanceToCreate