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

Fredrik <>
Mon Jul 1 11:23:45 CEST 2013

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:



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

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