interface WithNumber {
  foo: number;
}

interface WithString {
  bar: string;
}

type MyType = WithNumber | WithString;

interface Container<T extends MyType = MyType> {
  content: {
    value: T
  }
}

interface WithNumberContainer extends Container<WithNumber> { };
interface WithStringContainer extends Container<WithString> { };

type MysteryContainer = WithNumberContainer | WithStringContainer;

interface ParameterWithContainer<C extends MysteryContainer = MysteryContainer> {
  container: C
}

function isParameterWithNumberContainer(arg: ParameterWithContainer<MysteryContainer>): arg is ParameterWithContainer<WithNumberContainer> {
  return typeof (arg.container.content.value as WithNumber).foo === "number";
}

function test(arg: ParameterWithContainer<WithNumberContainer | WithStringContainer>) {
  if (isParameterWithNumberContainer(arg)) {
    arg.container.content.value.foo;
    return;
  }
  /*
   * Error:
   *   Property 'bar' does not exist on type 'MyType'.
   *     Property 'bar' does not exist on type 'WithNumber'.
   */
  arg.container.content.value.bar;
}