export type Writable<T> = {
    -readonly [K in keyof T]: T[K];
};

class Foo {
    readonly foo = 'foo';
}

class Bar {
    readonly bar = 'bar';
}

type WritableFooBar = Writable<Foo | Bar>;

declare function isFoo(obj: any): obj is Foo;

function test(foobar: WritableFooBar): void {
    if (!isFoo(foobar)) {} // happens also with if(isFoo())

    foobar
    // ^? Foo | Writable<Bar>
}