Intersection of objects are not extendable with more properties or even queryable about other properties

```js
/* @flow */

function mergeOptions (defaultOptions: A, options: ?B): A & B {
return {...defaultOptions, ...options};
}

const obj = mergeOptions({a: 'abc'}, {b: 'dee'});

obj.c = 'cee';
```

Gives an error like: `^ assignment of property `c`. Property cannot be assigned on any member of intersection type`.

https://flow.org/try/#0PQKgBAAgZgNg9gdzCYAoVUCuA7AxgFwEs5swBbAUwCcBzCgeQAciSBnMAHgEEAuegIwBWFAgBowAIT5CR+AHwAKACYUoAQ0wx8TFtlY8wXcXGbE9BgPwSAlAa5gAZJLABvVGDBUK+TFVIuAOiCVdU1tUzZxIICTXVYAXwBuVHj0XDZ8MDghMABecmo6HTNWBRc1AwByNX5cSvjxF34qlQp662TUbMEA3DywStwKNsSgA

What's the reason for this?

You can't even check if another property is defined or not:

```js
/* @flow */

function mergeOptions (defaultOptions: A, options: ?B): A & B {
return {...defaultOptions, ...options};
}

const obj = mergeOptions({a: 'abc'}, {b: 'dee'});

if (obj.c !== undefined) {
console.log('c is defined');
}
```