Taras said it's a bug in Nightly.
Downgrading to 16.0a1 (2012-06-05) fixes the problem.
I'm experiencing the same issue with the same build.
Range between the two Nightlies: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=a7a905fd70d5&tochange=6338a8988917
mozilla-central range: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=a7a905fd70d5&tochange=c76497029f0d Came from the inbound merge, so inbound range: https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=577a88fc97b6&tochange=b7fb3d52c843 Caused by bug 659577.
I've attempted a backout of the main bug 659577 landing and the followup commit, but there are conflicts with things that landed after, so think it's perhaps just safer to leave for Luke to take a look at for now.
Yes, I'll look at it immediately.
Phew, I've tracked it down to a bug in the mjit changes: increment on a formal argument when formals are aliased by 'arguments' (but not by the scope chain) doesn't increment the canonical slot in the arguments object and so is effectively a nop. Should be a simple fix; writing mjit patches isn't a good idea at this hour so I'll do it tomorrow.
Created attachment 630901 [details] [diff] [review] fix and test Actually, the fix is pretty simple.
Nightlies retriggered on 7e4c2abb9fc9.
Verified fixed with the new Nightly - thanks! :-) Mozilla/5.0 (Windows NT 6.1; WOW64; rv:16.0) Gecko/16.0 Firefox/16.0a1 http://hg.mozilla.org/mozilla-central/rev/7e4c2abb9fc9