[Cuis-dev] Bugfix: wrong bytecode when creating a class instance variable interactively

[Juan Vuletich]

Amazing!

On 2026-10-06 4:59 AM, Nicolas Cellier via Cuis-dev wrote:
> Gosh, I accidentally fixed it in Squeak !7 years before it was even 
> noticed...
> https://source.squeak.org/trunk/Kernel-nice.346.diff
>
> Le lun. 5 oct. 2026 à 22:16, Juan Vuletich via Cuis-dev 
> <cuis-dev at lists.cuis.st> a écrit :
>
>     Hi Facu,
>
>     Wow! Not every day you find a bug that is over 40 years old!
>
>     Just pushed this to GitHub. Thanks!
>
>     Cheers,
>
>     On 2026-10-04 1:01 PM, Facundo Javier Gelatti via Cuis-dev wrote:
>>     Hello!
>>
>>     This is a bug report + fix.
>>
>>     The bug is the following: declaring a class instance variable
>>     from the "Unknown variable" popup sometimes produces a method
>>     whose bytecode does not match the source (it stores into the
>>     wrong variables).
>>     I attach some code to see this (example.workspace.st
>>     <http://example.workspace.st>), which should be run from a workspace.
>>
>>     This happens because Metaclass>>addInstVarName: reverses the
>>     order of existing variables (which is already a weird behavior),
>>     and the compiler resolves instance variables to slot numbers at
>>     the start of the compilation. So, in an interactive environment:
>>     1. When we accept a method with an undeclared name, a
>>     notification is signalled mid-compilation.
>>     2. Then, from the popup, we choose to declare an instance variable.
>>     3. So, Parser>>declareInstVar: sends #addInstVarName:. This adds
>>     the instance variable (reversing the order of the existing ones),
>>     and triggers a recompilation of all existing methods (this does
>>     not include the method mid-compilation, which is not yet
>>     installed on the class).
>>     4. Then the compilation resumes. But the encoder has already
>>     assigned a slot index to every pre-existing variable, so the
>>     system ends up producing the wrong bytecode.
>>     Note that the class needs to have at least two existing class
>>     instance variables for this to happen (since reversing just one
>>     does nothing).
>>
>>     The fix is to make Metaclass>>addInstVarName: preserve the order
>>     of the variables (as Class>>addInstVarName: already does). With
>>     the order preserved, the indices the encoder computed before the
>>     declaration stay valid.
>>
>>     I've included tests for the bug, and also two additional things
>>     for consistency: one test for this same thing but instance-side,
>>     and a change to #removeInstVarName: to also preserve the order of
>>     the variables (removing class instance variables does not produce
>>     wrong behavior, but it is a bit confusing to see the order of the
>>     variables change).
>>
>>     * * *
>>
>>     As a side-note: I've been chasing this bug for some time, since
>>     it specially affected students using Cuis-University (denotative
>>     objects are classes behind the scenes, so all instance variables
>>     are class instance variables). The bug was hard to find because
>>     when you recompile the method (e.g. make any change and
>>     recompile, or file it out and then file it in, etc.), the
>>     bytecode is silently fixed.
>>
>>     Cheers!
>>     Facu
>>
>     -- 
>     Juan Vuletich
>     www.cuis.st <http://www.cuis.st>
>     github.com/jvuletich <http://github.com/jvuletich>
>     researchgate.net/profile/Juan-Vuletich <http://researchgate.net/profile/Juan-Vuletich>
>     independent.academia.edu/JuanVuletich <http://independent.academia.edu/JuanVuletich>
>     patents.justia.com/inventor/juan-manuel-vuletich <http://patents.justia.com/inventor/juan-manuel-vuletich>
>
>     -- 
>     Cuis-dev mailing list
>     Cuis-dev at lists.cuis.st
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>
>
-- 
Juan Vuletich
www.cuis.st
github.com/jvuletich
researchgate.net/profile/Juan-Vuletich
independent.academia.edu/JuanVuletich
patents.justia.com/inventor/juan-manuel-vuletich
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