interface A {
  foo : number;
}

function isA(object: any): object is A {
  return true;
}

let a: A | boolean;

if (isA(a)) {
  a.foo;   // compiles
}



interface B {
  foo? : number;
}

function isB(object: any): object is B {
  return true;
}

let b: B | boolean;

if (isB(b)) {
  b.foo;  // does not compile because type inference says its `B | boolean`
}