interface AB {
  A: string;
  B?: string;
}

const childObject: AB = {
  A: "test"
}

const parentObject = {
  childObject
};

function function1({childObject: {A, B = ""}}) {
  console.log(A);
  console.log(B);
}

function function2({childObject: {A, B = ""} = {A: "", B: ""}}) {
  console.log(A);
  console.log(B);
}

function function3({childObject: {A, B} = {A: "", B: ""}}) {
  console.log(A);
  console.log(B);
}

/* Test #1: Compiles cleanly */
function1(parentObject);

/*
  Test #2: Gives expected error.
  Error: Argument of type '{}' is not assignable to parameter of type '{ childObject: { A: any; B?: string; }; }'. Property 'childObject' is missing in type '{}'.
*/
function1({});

/*
  Test #3: Not sure why this one is giving an error.  A default is provided for B if the object exists, so
  technically B is not required.  I would have only expected to see this error on Test #5 below.
  Argument of type '{ childObject: AB; }' is not assignable to parameter of type '{ childObject?: { A: string; B: string; }; }'. Types of property 'childObject' are incompatible. Type 'AB' is not assignable to type '{ A: string; B: string; }'.
  Property 'B' is optional in type 'AB' but required in type '{ A: string; B: string; }'.
*/
function2(parentObject);

/* Test #4:  Compiles cleanly */
function2({});

/*
  Test #5: Gives expected error.  (Same as #3 but this one makes sense.)
  Argument of type '{ childObject: AB; }' is not assignable to parameter of type '{ childObject?: { A: string; B: string; }; }'. Types of property 'childObject' are incompatible. Type 'AB' is not assignable to type '{ A: string; B: string; }'.
  Property 'B' is optional in type 'AB' but required in type '{ A: string; B: string; }'.
*/
function3(parentObject);

/* Test #6: Compiles cleanly */
function3({});