Example of flow disjoint union checking not carrying through closures

flow-disjoint-union-checking.js

// @flow

type A = {
  lower: 'a'
}

type B = {
  lower: 'b',
  bMethod: Function
}

type Alphabet = A | B

const method1 = (alphabet: Alphabet) => {
  if (alphabet.lower === 'b') {
    alphabet.bMethod(); // this is okay
  }
};

const method2 = (alphabet: Alphabet) => {
  if (alphabet.lower === 'b') {
    return () => {
      alphabet.bMethod(); // this is not okay
    };
  }
};

Error message:

src/flow-disjoint-union-checking.js:23
 23:       alphabet.bMethod(); // this is not okay
                    ^^^^^^^ property `bMethod`. Property not found in
 12: type Alphabet = A | B
                     ^ object type