Interface extending Array loses all type safety

[massimonewsuk]

interface NumberArray extends Array<number> { }

function doSomething(x: NumberArray | null) {
    if (Array.isArray(x)) {
        printString(x); // Why does this work?
        printNumber(x); // Why does this work?
        printBoolean(x); // Why does this work?
        printArray(x);
    }
}

function printString(x: string) {
}

function printNumber(x: number) {
}

function printBoolean(x: boolean) {
}

function printArray(x: Array<any>) {
}

If I use type NumberArray = Array<number>; then everything works as expected.

I tried this in the TypeScript playground and the errors show up as expected. I tried reading Flow's documentation on interfaces but I couldn't find any reason why the declaration using interface shouldn't work (versus type).

This issue might be related to #5759 where the interface keyword acts differently to the type keyword.

[massimonewsuk]

Still an issue in v0.67.0

[massimonewsuk]

Still an issue in v0.70.0

[Mocha--]

Guys, any update for this issue?

I got the same severe problem in my code. All properties are lost, which are supposed to be inherited from its parent interface.