Template literals are improperly being coerced

[sebmck]

According to 12.2.8.5 template literal expressions should be coerced to strings.

Let sub be the result of evaluating Expression.
Let middle be ToString(sub).

var x = { toString: function () { return "foo"; }, valueOf: function () { return "bar"; } };
console.log(`${x}`);

Actual:

var x = { toString: function () { return "foo"; }, valueOf: function () { return "bar"; } };
console.log(("" + x));

Expected:

var x = { toString: function () { return "foo"; }, valueOf: function () { return "bar"; } };
console.log(("" + String(x)));

Relevant V8 bug report here.

[arv]

If you've been following the discussion on the PR it is not clear we care about fixing this. To fix this we need to make sure that we throw for symbols which makes the generated code pretty ugly (and slower).

Maybe Traceur should follow Babel and use a spec/loose mode?

[sebmck]

I think that's a good approach. I'm concerned about performance too since template literals are almost certainly going to appear in hot code. Not sure whether it's a good idea to do it by default though. Perhaps it can be considered the same as the typeof symbol rewriting (opt-in)?