Properties of instances of anonymous classes have type <any> after an instanceof guard.

TypeScript Version: 2.4.1

Code

function create_my_class<T>() {
    return class {
        value: T;
        constructor(
            value: T
        ) {
            this.value = value;
        }
    }
}
const MyClassWithNumber = create_my_class<number>();
const with_number = new MyClassWithNumber(234);

with_number.value = 10;
// with_number.value = "sdf"; // error, and rightly so

const something: any = with_number;

if(something instanceof MyClassWithNumber) {
    something.value = 20;
    something.value = "sdf"; // not an error, but should be one
}

Expected behavior:
The last assignment should be marked as a type error by TS.

Actual behavior:
The last assignment is not marked as error. This is somewhat surprising, as the line

with_number.value = "sdf";

is righteously reported as a type error by TS.

Not a huge issue, but prevented me from using a class factory.

Start with the supplied TypeScript reproduction and trace how the instanceof guard narrows the value returned by the generic class factory. Verify the behavior using the shown assignments, then ensure the guarded string assignment is reported as a type error while the numeric assignment remains valid.