Control Flow Analysis + Getter Properties

[mariusschulz]

TypeScript Version: 2.0.3 / nightly (2.1.0-dev.20161106)

The following example doesn't compile without errors in the most recent TypeScript nightly build. Control flow analysis determines that scanner.current has the type "/", and thus TypeScript flags the string comparison with "*". However, it doesn't take into account that current is a getter property, which returns a different value after advance() has been called.

Wouldn't it make sense to treat getter properties like methods in this case and not narrow their type to a string literal type?

Code

class Scanner {
    constructor(private input: string, private pos = 0) {}

    advance() {
        this.pos++;
    }

    get current() {
        return this.input[this.pos];
    }
}

const scanner = new Scanner("/* Comment */");

if (scanner.current === "/") {
    scanner.advance();

    if (scanner.current === "*") {
        // Error
        // Operator '===' cannot be applied to types '"/"' and '"*"'
    }
}

Expected behavior:
No errors.

Actual behavior:
Error Operator '===' cannot be applied to types '"/"' and '"*"'

[aluanhaddad]

This is not really related to the property being computed or even to its being a property at all. Calling any function could have arbitrary side effects that cannot plausibly be tracked. Here is a minimal repro

let current = "/* comment */";

const advance = () => {
    current = current.slice(1);
};

if (current === "/") {

    advance();

    if (current === "*") { // [ts] Operator '===' cannot be applied to types '"/"' and '"*"'
    }
}

Edit: see #9998

[mariusschulz]

Thank your for linking to #9998. For interested readers, Anders precisely answers my questions here. Interestingly, the same issue came up in the TypeScript compiler (advancing to the next token).

tl;dr: It's a calculated tradeoff of control flow analysis.