Erroneous control flow type narrowing in loops

TypeScript Version: 2.3.2

Code

const enum Escape {None, Any}

function scan(): void {
    let escape: Escape = Escape.None;

    while (true) {
        // Error: Operator '===' cannot be applied to types 'Escape.None' and 'Escape.Any'.
        escape = escape === Escape.Any ? Escape.None : Escape.Any;
    }
}

Expected behavior:

The type to be correctly broadened, since it's in a loop.

Actual behavior:

The error denoted in the code comment above the line in question.

Note that neither of these variants reproduce the bug:

// Variant 1
const enum Escape {None, Any}

function scan(): void {
    let escape = Escape.None as Escape;

    while (true) {
        escape = escape === Escape.Any ? Escape.None : Escape.Any;
    }
}

// Variant 2
function scan(): void {
    let escape = false;

    while (true) {
        escape = escape === true ? false : true;
    }
}

My current workaround is casting the escape in the condition, like in this:

const enum Escape {None, Any}

function scan(): void {
    let escape: Escape = Escape.None;

    while (true) {
        // No error here
        escape = (escape as Escape) === Escape.Any ? Escape.None : Escape.Any;
    }
}

Also, note that in my code, I have about 8-10 cases, for a while/switch-based finite state machine loop.

I just noticed what I believe is the same bug, but without the loop:

enum MyEnum {
    Foo,
    Bar
}

class MyClass {
    private state: MyEnum;

    // Set state to Bar
    private modify(){
        this.state = MyEnum.Bar;
    }

    public fn() {
        this.state = MyEnum.Foo

        this.modify();

        if (this.state === MyEnum.Bar) {
            // Do stuff here...
        }
    }
}

This produces the following error

tmp.ts(19,13): error TS2365: Operator '===' cannot be applied to types 'MyEnum.Foo' and 'MyEnum.Bar'.

Just tested this with tsc version 2.7.2

Seems that typescript doesn't think that variables of enum type can ever change?

Josh (@JShabtai)

Josh (@JShabtai) Yours is subtly different:

Mine is purely with local variables, where yours is with properties.
Yours could change between checks, where mine theoretically couldn't.

Works now. Other comment is a dupe of 9998

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Erroneous control flow type narrowing in loops #15835

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Erroneous control flow type narrowing in loops #15835

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dead-claudia commented on May 14, 2017

JShabtai commented on Mar 15, 2018

dead-claudia commented on Mar 16, 2018

RyanCavanaugh commented on Mar 5, 2024

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