strictNullChecks: false positive with switch in for loop

TypeScript Version: 3.1.0-dev.20180920

Search Terms: strictNullChecks

Code
This might seem weird, but is based on actual real-world code.

declare function baz(): Promise<true | "a" | "b" | "c" | Error>;

async function foo() {
    let bar: "x" | Error;
    for (let i = 0; i < 3; i++) {
        const test = await baz();
        switch (test) {
            case true: return;
            case "a": throw new Error("a");
            case "b": {
                bar = "x";
                continue;
            }
            default: {
                if (test instanceof Error) {
                    bar = new Error("foo");
                    continue;
                } else {
                    throw new Error("c");
                }
            }
        }
    }
    // false positive: bar is used before being assigned
    if (bar === "x") {

    } else {
        bar.message = "something";
    }
}

Expected behavior:
No error after the for loop. The only way to get there is if bar has the value "x" or is an instance of Error. If baz resolves to true, the function returns, "b" sets bar to "x", and "a" and everything that is not an Error (including "c" throw, and an Error sets it to an Error.

Actual behavior:
Variable "bar" is used before being assigned.

Related Issues: #24091

We don't check that the for loop is run at least once.

Would you consider adding a way to assert "I definitely know this for loop will be run at least once"?

Andy (Andrewkraft) (@Andy-MS)

Andy (Andrewkraft) (@Andy-MS) It seems I forgot to mention a key point. If I change the condition after the loop to the following

    if (bar! === "x") {
        // bar is "x"
    } else {
        // bar is "x" | Error, should be Error
        bar.message = "something";
   }

the type for bar in the else branch includes "x", when it should be an Error instance. The analyzer gets it right when I inline the loop body or do this:

    bar = bar!; // <-- gross!
    if (bar === "x") {
        // bar is "x"
    } else {
        // bar is Error
        bar.message = "something";
   }

What about something like this to tell the compiler to treat the loop body as "inlined"?

for!(...) { // notice the !
    // ...
}

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

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strictNullChecks: false positive with switch in for loop #27239

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strictNullChecks: false positive with switch in for loop #27239

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AlCalzone commented on Oct 31, 2018

AlCalzone commented on Nov 1, 2018

typescript-bot commented on Dec 14, 2018

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