Destructuring function param doesn't work with union types

[atabel]

Here is an example with a redux reducer:
This code:

type FooAction = { type: "FOO", payload: Object };
type BarAction = { type: "BAR", payload: Object };

type Action = FooAction | BarAction;

function reducer(state, { type, payload }: Action) {
  switch (type) {
    case "FOO": return { ...state, value: payload };
    case "BAR": return { ...state, value: payload };
    default:
      return state;
  }
}

Produces this error:

6: function reducer(state, { type, payload }: Action) {
                             ^ string literal `BAR`. Expected string literal `FOO`, got `BAR` instead
6: function reducer(state, { type, payload }: Action) {
                             ^ string literal `FOO`
6: function reducer(state, { type, payload }: Action) {
                             ^ string literal `FOO`. Expected string literal `BAR`, got `FOO` instead
6: function reducer(state, { type, payload }: Action) {
                             ^ string literal `BAR`
8:     case "FOO": return { ...state, value: payload };
            ^ string literal `FOO`. This === check always fails because `FOO` is not the same string as `BAR`
6: function reducer(state, { type, payload }: Action) {
                             ^ string literal `BAR`

see in Try Flow

But, if I move destructuring to the function body instead of function declaration:

function reducer(state, action: Action) {
  const { type, payload } = action
  switch (type) {
    case "FOO": return { ...state, value: payload };
    case "BAR": return { ...state, value: payload };
    default:
      return state;
  }
}

no errors!

see in Try FLow

[kristiehowboutdat]

Here is another example using Flow 0.73.0: Try Flow

Filed #6408