Using of rest argument binding in inner function, breaks it

[mstoykov]

Original report grafana/k6#2131.

Smallest reproduction:

function f(...rest) {
         () => rest; // remove this line to fix.
         return rest.length;
}
f(1,2) // this returns 0 not 2

as shown there having adding another variable to which to using rest and then using that in the lambda fixes it including that the following works:

function f(...rest) {
         const alias =  rest;
         return (() => alias.length)();
}
f(1,2) // this returns 2 actually

but doesn't work if in this case you just use rest instead.

This doesn't seem related to lambdas as it also doesn't appear to be working with functions.

I would guess some optimizations somewhere as calling eval before the lambda fixes it.

Another thing that I noticed (that might be a completely separate issue) is that

function f(...rest) {
         eval(rest[0]) // not the missing ;
         () => rest; 
         return rest.length;
}
f(1,2) // this returns 0 not 2

returned Line 3:3 Malformed arrow function parameter list but I don't know if this isn't really how it should work 🤷

[dop251]

Thanks for reporting. The other issue that you mention is not a bug, ASI does not apply to this case, so it's interpreted as eval(rest[0])() => rest