The type checking doesn't work well in asynchronous code.

[fenying]

TypeScript Version: 2.2.2 / nightly (2.2.0-dev.201xxxxx)

Code

enum ETest {
    A,
    B,
    C
}

let g: ETest;

function test_a(): void {

    g = ETest.B;
}

(async () => {

    g = ETest.A;

    await new Promise(function(resolve, reject): void {

        setTimeout(resolve, 1000);
    });

    if (g === ETest.B) { // here will be marked as an error

        console.log("hey");
    }

})();

test_a();

Expected behavior:

That line shouldn't be marked as an error.

Actual behavior:

TS-compiler shows an error like this:

error TS2365: Operator '===' cannot be applied to types 'ETest.A' and 'ETest.B'.
16:33:07 - Compilation complete. Watching for file changes.

---

Maybe the strict type checking should be disabled for closured variables or this.xxxx?

[HerringtonDarkholme]

This is related to how control flow analysis handles async/generator functions. They need special handling such as #11928.

However, code blocks in async/generator function are handled as if they were synchronous. This is pretty sweet if variable g is confined in async function, that is, g is only mutated inside async function, but it also breaks your code. We can also handle async/generator function as truly "async". But it breaks other inferences. For similar discussion, see #9998. I believe there is no silver bullet.

In this particular example, I would suggest finding a workaround which avoids changing g. Or, you can write a get_g function.

enum ETest {
    A,
    B,
    C
}

let g: ETest;

function test_a(): void {
    g = ETest.B;
}

function get_g() {
    return g
}

(async () => {

    g = ETest.A;

    await new Promise(function(resolve, reject): void {

        setTimeout(resolve, 1000);
    });

    if (get_g() === ETest.B) { // here will be marked as an error

        console.log("hey");
    }

})();

test_a();