Type narrowing of string literal seems to get loosened in callbacks

TypeScript Version: 2.2.1

Code

function myValueArg(arg: "value") { }

function myStringArg(arg: string) { }

namespace ns { // Not needed, but shows it's not scope related
    let str = "value";

    if (str === "value") {
        myValueArg(str);
        [].forEach(() => {
            myValueArg(str); // Argument of type 'string' is not assignable to parameter of type '"value"'.
        });
    } else {
        myStringArg(str);
        [].forEach(() => {
            myStringArg(str);
        });
    }
}

Expected behavior:
No warnings: the value (and type) of str has not changed.

Actual behavior:
Warning Argument of type 'string' is not assignable to parameter of type '"value"'.

Use const. let bindings are reset because the function might be invoked asynchronously and a new value could be set to the let variable in the mean time.

That sort of makes sense (though this is just a sample, the real code I noticed it on is most definitely not a constant). Wouldn't it be better to annotate things that are definitely not asynchronous to prevent this spurious warning?

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

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Type narrowing of string literal seems to get loosened in callbacks #14748

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Type narrowing of string literal seems to get loosened in callbacks #14748

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gcnew commented on Mar 21, 2017

Rycochet commented on Mar 21, 2017

typescript-bot commented on Nov 16, 2017

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