Type narrowing of string literal seems to get loosened in callbacks

[Rycochet]

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"'.

[gcnew]

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.

[Rycochet]

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?

Duplicate of #9998, also #11498.

[typescript-bot]

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