Lua comma evaluation order issue? tfound[1],num = num, num - 1 breaks on luau sometime after 0.541

[SteveDE]

We have some binary search copy pasta from here http://lua-users.org/wiki/BinarySearch

It has this pesky line:
tfound[1],num = num, num - 1

In 0.541 this line emits these opcodes, and everything works as it did on stock Lua:

   41:                 tfound[1],num = num, num - 1
41: MOVE R11 R10
41: SUBK R12 R10 K13
41: SETTABLEN R11 R9 1
41: MOVE R10 R12

In 0.554, it emits this, which breaks our search:

   41:                 tfound[1],num = num, num - 1
41: SUBK R10 R10 K13
41: SETTABLEN R10 R9 1

If I remove the comma, it works again. The order is correct and is nicely optimized as 0.554

   39:                 tfound[1] = num
39: SETTABLEN R10 R9 1
   40:                 num = num - 1
40: SUBK R10 R10 K13

I'm ready to believe this is ambiguous or a bug in the original, but I thought I should mention it for compatibility reasons.

[vegorov-rbx]

I can confirm that this is an issue with multiple-assignment in Luau compiler that's related to a not-so-recent optimization that we've added.
We're now working on a fix for this case.

[vegorov-rbx]

We have a fix ready internally, next release is planned for Dec 2nd.

[SteveDE]

Fast! Thank you very much!

[SteveDE]

Merged to 0.555 and the byte code generate is unfortunately in the wrong order still

22: tfound[1],num = num,num - 1
22: SUBK R10 R10 K4
22: SETTABLEN R10 R9 1