Assertion function error messages are unactionable and unclear

[DanielRosenwasser]

class Foo {
    bar?: Foo

    public setBar(): asserts this is Foo & {bar: Foo}  {
        this.bar = this;
    }

    public sayHi(): string {
        return 'Hi!';
    }
}

function test() {
    let foo = new Foo();

    foo.bar = undefined;

    foo.setBar();

    if (foo.bar) {
        console.log('This should be OK: ' + foo.bar.sayHi());
    }
}

Current error

Assertions require every name in the call target to be declared with an explicit type annotation.

'foo' is declared here.

foo is declared here but... but okay so what am I supposed to do now??

[threehams]

Is it possible this would make arrow function assertion errors even more confusing, given their use it marked as a design limitation in #34523? This example would (I think?) result in the error constAssert requires an explicit type annotation, which is still unclear.

function funcAssert (condition: boolean): asserts condition {
    if (!condition) {
        throw new Error();
    }
}

const constAssert = (condition: boolean): asserts condition => {
    if (!condition) {
        throw new Error();
    }
}

(thing: number | null) => {
    funcAssert(!!thing); // this is fine
    thing;
}
(thing: number | null) => {
    constAssert(!!thing); // why doesn't this work, and what am I supposed to do?
    thing;
}

https://www.typescriptlang.org/play/?ssl=21&ssc=1&pln=1&pc=1#code/GYVwdgxgLglg9mABKSBBAzugpgJyogCggQBMZYEAuRAIzjgBssBDMASmuc1ynUWLBkKSAN4AoRJMQxghAIQCh8donFT1iKAAsccAO6IwWAwFEcunATYBuCVIC+Yx2IHp8rqBmx5EAXkKK5MrUdIws7JzceHyBwn4AfKp2kjLyscpsSRpS2roGRqbmcJY2yYiOzgTaMGAA5tRgIAC2NLiIAD6GIAwMmb6JalIoEF48BHJy1XU2iAD0s5paMHzLyDVYZVO1to5VS3UNza04HV09fQNlHqN445P7tTPziHpaAJ6IJHBY6GAA5Phqnw9MUANYAGkQrBILy0zHwzCaiAAkoh0CAAA4YuDYGFQOCfOAAfk2Dx2YiAA