Trying to use disjoint unions in Flow

flow-recursion-limit.js

/* @flow */
// Modeled after https://github.com/facebook/flow/issues/1664#issuecomment-222934455
type AnimalT = {
  id: number,
  name: string
};
type CatT = AnimalT & {
  isPurring: bool
};
type DogT = AnimalT & {
  isBarking: bool
};


// This is cool.
const garfield:AnimalT = {
  id: 42,
  name: 'Garfield',
  isPurring: true
};
console.log(garfield);

// This is cool.
const rex:DogT = {
  id: 45,
  name: 'Rex',
  isBarking: false
};
console.log(rex);

// Trying to check for a property and then work with a more
// specific type doesn't seem to work.
if (garfield.isPurring) {
  const cat:CatT = garfield;
  console.log(cat);
}

// Nor does the approach in https://github.com/facebook/flow/issues/1664#issuecomment-222934455
if (garfield.isPurring) {
  (garfield:CatT);
  console.log(garfield);
}

Interestingly, the switch statement works and also detects the type error:

switch (garfield.type) {
  case 'cat': console.log(garfield.isPurring); break;
  case 'dog': console.log(garfield.isBarking); break;
}

and changing them both to isBarking yields:

 51:   case 'cat': console.log(garfield.isBarking); break;
                                        ^^^^^^^^^ property `isBarking`. Property not found in
 51:   case 'cat': console.log(garfield.isBarking); break;
                               ^^^^^^^^ object type