[erlang-bugs] Compiler bug with unsafe variables in nested try statements.

[Paul Davis]

Thanks!

On Tue, Jul 23, 2013 at 2:28 AM, Anthony Ramine <n.oxyde@REDACTED> wrote:
> Hello Paul,
>
> I've added a fix for that bug in another patch I submitted which fixes a bunch of problems with variable usage tracking.
>
> Regards,
>
> --
> Anthony Ramine
>
> Le 21 juil. 2013 à 00:33, Paul Davis a écrit :
>
>> I seem to have uncovered a bug when detecting or displaying "variable
>> 'Acc' unsafe in 'try'" errors. I managed to reduce this to a fairly
>> small test case:
>>
>> -module(bam).
>> -export([bang/0]).
>> bang() ->
>>    case 1 of
>>        nil ->
>>            Acc = 2;
>>        _ ->
>>            try
>>                Acc = 3,
>>                Acc
>>            catch _:_ ->
>>                ok
>>            end
>>    end,
>>    Acc.
>>
>> The important point that I found is that its necessary to reference
>> Acc in the try body after its been bound. Without that the unsafe
>> error is generated properly.
>>
>> The error I'm seeing is:
>>
>> Function: bang/0
>> bam.erl:none: internal error in core_dsetel_module;
>> crash reason: {{badmatch,error},
>>               [{sys_core_dsetel,visit,2,
>>                                 [{file,"sys_core_dsetel.erl"},{line,98}]},
>>                {sys_core_dsetel,visit,2,
>>                                 [{file,"sys_core_dsetel.erl"},{line,117}]},
>>                {sys_core_dsetel,visit_module_1,3,
>>                                 [{file,"sys_core_dsetel.erl"},{line,79}]},
>>                {sys_core_dsetel,visit_module,1,
>>                                 [{file,"sys_core_dsetel.erl"},{line,75}]},
>>                {sys_core_dsetel,module,2,
>>                                 [{file,"sys_core_dsetel.erl"},{line,70}]},
>>                {compile,core_dsetel_module,1,
>>                         [{file,"compile.erl"},{line,1159}]},
>>                {compile,'-internal_comp/4-anonymous-1-',2,
>>                         [{file,"compile.erl"},{line,276}]},
>>                {compile,fold_comp,3,[{file,"compile.erl"},{line,294}]}]}
>>
>> I discovered this on R14B01 but its still present on R16B01 as well.
>>
>> Out of curiosity I also reproduced it with an if statement as well:
>>
>> #!/usr/bin/env escript
>>
>> -mode(compile).
>>
>> main(_) ->
>>    Maybe = bar(),
>>    if Maybe ->
>>        Acc = 1;
>>    true ->
>>      try
>>          Acc = 2,
>>          Acc %, throw(foo)
>>      catch _ ->
>>        ok
>>      end
>>  end,
>>  erlang:display(Acc).
>>
>> bar() -> false.
>>
>> Somewhat fun is that if we comment out -mode(compile) for interpreted
>> mode we can get all three values (1, 2, unbound) of Acc by changing
>> bar/0 and/or uncommenting the throw statement.
>>
>> Obviously my code is broken trying to reference a variable like that;
>> I was just a bit surprised to see a compiler error.
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>