885463 - Warn about 'yield' without operand

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jason Orendorff [:jorendorff]]

In a generator, we parse

    yield
        f();

as two statements, meaning

    yield undefined;
    f();

But in ES6, this will parse as a single statement:

    yield f();

Users should be warned.

(We may not break their code, at least not immediately, since ES6 generators are distinguished from JS1.7+ generators by the "*". But people will naturally want to port up to ES6, and we don't want them to hit silent breakage. So a warning is in order.)

Comment 1

[Jason Orendorff [:jorendorff]]

Steal this bug if you have time. I'm going to try and get this into FF24 though.

Comment 2

[Jason Orendorff [:jorendorff]]

Attachment: v1

Comment 3

[Jeff Walden [:Waldo]]

Comment on attachment 765663 [details] [diff] [review]
v1

Review of attachment 765663 [details] [diff] [review]:
-----------------------------------------------------------------

Awesome.  Land it!

::: js/src/jit-test/lib/asserts.js
@@ +72,5 @@
> +                print("assertWarning: " + msg);
> +            throw exc;
> +        } finally {
> +            if (!hadWerror)
> +                options("werror");

Guh, options() is not a good interface.

Comment 4

[Jason Orendorff [:jorendorff]]

No kidding. Filed bug 885766.

Comment 5

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/818298a5a183

Comment 6

[Olli Pettay [:smaug][[e-mail]]]

(Got here from Bug 891806)

Why do we need yield undefined; and not just yield; ?
I can understand 
yield 
  f(); needing some change, but will yield; mean something totally 
different than what it is currently?

Comment 7

[Jason Orendorff [:jorendorff]]

Unless the proposal changes, `yield;` will be a SyntaxError. We plan to implement the standard (soon) and remove the SpiderMonkey-only extensions (eventually).

smaug, I encourage you to take this to es-discuss! If "yield undefined" looks ugly in Moz-only code, then it'll look just as ugly in Web code, which will follow the new standard syntax.

Since you have existing real-world code, you are in a position to make a uniquely convincing argument. You've seen the future, you've lived there, you're proposing a simple feature that makes the future prettier.

Individuals can make a difference on TC39. Please give it a shot if you really care about this syntax.

Comment 8

[Kohei Yoshino]

Added: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/25/Site_Compatibility