`in` keyword doesn't seem to narrow types

[massimonewsuk]

Try pasting the following code into https://flow.org/try/

interface Foo {
    kids: any[];
}
interface Bar {
    category: "bar";
    children: any[];
}
interface Baz {
    category: "baz";
    minions: any[];
}

for (let item of ([] : Array<Foo | Bar | Baz>)) {
    if ("category" in item) {
       	// expecting item to be Bar | Baz here, or at least something with the property "category"
        item.category
    }
}

This code example works as expected in the TypeScript playground (provided you change the casting of the array literal using : to as, as TS uses different syntax for casting)

[massimonewsuk]

Still an issue in v0.67.0

[massimonewsuk]

Still an issue in v0.70.0

[gkz]

It does now when you use exact object types (it is unsafe to do so with interfaces as they may be extended to add that property): https://flow.org/try/#1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAztvhgE6UYCe+JADpdhgCYowa5kA0I2KAFcAtiRQAXSkOz9sADwxgJ+NPTbYuQ3BMnTZA+Y2yU4IwRO4A6SFBIrGVDGM7c+h46fNRLuKxJIGWh8MeT0ZfhYlCStpHzNsFBAMIQkIEQwJODAQfiEyfBE4eWw2fDgofDBMsAALfAA3KjgsXGxxZC4eAw0G-GhcWn9aY3wWZldu-g1mbGqJUoBaCRHEzrcDEgBrbAk62kXhXFxJ923d-cPRHEpTgyEoMDaqZdW7vKgoOfaSKgOKpqmDA+d4gB5fMA-P6LCCMLLQbiLOoYCqgh6-GDYRYIXYLSgkRZkCR4jpddwPfJLZjpOBkO4AX34kA0SRWxgABAAxYjsgC87OAAB0oOzReytnA2CRkOzgbQANoAXQA3ML6cK2dh2QAhKh8gXCsXs+bYBAQSi0GWCkCYSjW1UisV1OC4NgsKAyuVKh3qnyrHUYABe+qFjtFJrNFqtNqD9sNYqKUDg0GlsqgCpVauFwpg5vZAAo2hJ2XAFiJ2RAYAWleyZQBBah0AA83Ig7IAPgHKB2A4GAHwASgHBrDJar+etEfNtGtJZFpewImHoaNYsFEgA9Bv2QpjMoKggS2X2Wl2Tguz3dcHaiZZBXu5l2c97OySOldrUD+yAO6l2onm92WpTwVnZSdMlNadrXjVcj0XGwIMjGdR19I16VyEAGhMEhkygJIGgABisAAmAA2ABmKwCJAekgA