[erlang-bugs] [erlang-patches] Compiler/linter bug breaking unused variable warnings

[Anthony Ramine]

Hi Fredrik,

I've amended the commit message and gave an example of a simpler expression triggering the bug, e.g.:

t(X, Y) ->
  {A=X,A=Y}.

As for the vt_no_unused/1 call in expr_list/1, it is a mistake as Vt1 comes from the variable tables returned by expr/3 in the lists:foldl/3 call, which means it can't contain any old variable which was unused anyway.

Regards,

-- 
Anthony Ramine

Le 1 juil. 2013 à 11:23, Fredrik a écrit :

> On 06/06/2013 01:47 AM, Anthony Ramine wrote:
>> Hello,
>> 
>> When analyzing complex expressions (i.e. comprehensions, cases, tries, ifs and receives), erl_lint does not forget about old unused variables when returning the updated variable table. This causes a bug where old
>> unused variables are not recorded as such:
>> 
>> t(X, Y) ->
>>   #r{a=[ K || K <- Y ],b=[ K || K <- Y ].
>> 
>> As erl_lint uses vtmerge_pat/2 to merge the results of the analysis of the two list comprehensions, X is marked as used and the warning is not emitted.
>> 
>> The function vtmerge_pat/2 is used instead of the similar vtmerge/2 which does not mark multiple occurrences of a variable as usage to handle cases like the following one:
>> 
>> t(X, Y) ->
>>   #r{a=A=X,b=A=Y}.
>> 
>> Other simpler expressions like conses, tuples and external fun references does not correctly follow this behaviour.
>> 
>> This patch fixes both issues and makes erl_lint not return old unused variables in updated tables and makes all compound expressions use vtmerge_pat/2.
>> 
>> 	git fetch 
>> https://github.com/nox/otp.git
>>  fix-erl_lint-variable-usage
>> 
>> 	
>> https://github.com/nox/otp/compare/erlang:maint...fix-erl_lint-variable-usage
>> 
>> 	
>> https://github.com/nox/otp/compare/erlang:maint...fix-erl_lint-variable-usage.patch
>> 
>> 
>> Regards,
>> 
>> 
> Hi Anthony,
> Your patch has been reviewed and the feedback was:
> 
> "
> Hi,
> 
> The first example of the commit messages lacks a '}' before the dot.
> 
> I'm not sure I understand exactly what the paragraph starting with "Other simpler expressions..." means. Could you clarify, please? (And substitute "do" for "does"...)
> 
> I noticed that if the call to vt_no_unused() in the last line of expr_list/3 is removed the test suite still passes; there is no case where the call actually removes anything. Would it be possible to include a test that covers this particular call?
> 
> Best regards,
> 
> "
> 
> -- 
> 
> BR Fredrik Gustafsson
> Erlang OTP Team
>