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.

提供された TypeScript の再現コードから始め、instanceof ガードによってジェネリックなクラスファクトリが返す値の型がどのように絞り込まれるかを追跡します。示されている代入を使って動作を検証し、そのうえで、ガードされた文字列の代入が型エラーとして報告される一方、数値の代入は引き続き有効であることを確認します。