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digitalmars.D.bugs - [Issue 6683] New: Skipping declaration with goto resulted in nonsense

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=6683

           Summary: Skipping declaration with goto resulted in nonsense
           Product: D
           Version: unspecified
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: minor
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: destructionator gmail.com


--- Comment #0 from Adam D. Ruppe <destructionator gmail.com> 2011-09-16
16:53:35 PDT ---
I'm in the middle of a project right now so will be brief and can add more
later.

Given code like such:

====
auto somethingResult = getSomething();

if(somethingResult.empty)
       goto dont_have_something;

auto something = somethingResult.front;

dont_have_something:

  if(something !is null) {
        // potential problem here if the result was empty - "something" doesn't
refer to what I thought it would!
  }

=======

The goto over the variable declaration probably should have been a compile
error, I believe, from the spec. This was kinda a pain to find when it started
crashing since the variable actually referred to an entirely different object!

I'm using last month's dmd too, so possible it's already been fixed. I just
want to post something here before I forget about it.

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next sibling parent d-bugmail puremagic.com writes:
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Simen Kjaeraas <simen.kjaras gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |simen.kjaras gmail.com


--- Comment #1 from Simen Kjaeraas <simen.kjaras gmail.com> 2011-09-16 17:23:19
PDT ---
The spec says "It is illegal for a GotoStatement to be used to skip
initializations.". (
http://www.d-programming-language.org/statement.html#GotoStatement )

According to the glossary
(http://www.d-programming-language.org/glossary.html), "A code construct is
illegal if it does not conform to the D language specification. This may be
true even if the compiler or runtime fails to detect the error."

So the compiler is free to ignore illegal code. A future compiler, potentially
not dmd, may indeed flag it as a compile-time error, so any behavior caused by
illegal code should be regarded as unpredictable and unreliable.

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--- Comment #2 from Adam D. Ruppe <destructionator gmail.com> 2011-09-16
17:38:52 PDT ---
Is that saying "bugs in the compiler don't override the spec" or "goto over
initializations at your own risk"?

If it's the latter, this isn't a bug at all. (I can see how the compiler
throwing an error on this could get annoying, so I suppose it could go both
ways.)

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--- Comment #3 from Simen Kjaeraas <simen.kjaras gmail.com> 2011-09-16 17:47:46
PDT ---
It's the latter. Not only do you run the risk of your variables being garbage,
but your code might not compile on a future compiler. (not that this has ever
*not* been a problem with D, mind)

The compiler is free to issue an error or not, but the code is definitely
wrong.

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Kasumi Hanazuki <k.hanazuki gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |DUPLICATE


--- Comment #4 from Kasumi Hanazuki <k.hanazuki gmail.com> 2011-10-26 12:06:07
PDT ---
*** This issue has been marked as a duplicate of issue 602 ***

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Stewart Gordon <smjg iname.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |accepts-invalid
                 CC|                            |smjg iname.com


--- Comment #5 from Stewart Gordon <smjg iname.com> 2011-10-26 14:11:20 PDT ---
(In reply to comment #1)

 The spec says "It is illegal for a GotoStatement to be used to skip
 initializations.". (
 http://www.d-programming-language.org/statement.html#GotoStatement )
 
 According to the glossary
 (http://www.d-programming-language.org/glossary.html), "A code construct is
 illegal if it does not conform to the D language specification. This may be
 true even if the compiler or runtime fails to detect the error."


For all I know, that probably means "This may be true even if some compiler,
because of a bug in it, fails to detect the error."


 So the compiler is free to ignore illegal code.


Which is the bit of illegal code - the goto statement or the declaration it
skips?  In my experience, the compiler doesn't ignore either.

Moreover, why does the spec bother to forbid anything if compiler writers are
free to ignore it all?


 A future compiler, potentially
 not dmd, may indeed flag it as a compile-time error, so any behavior caused by
 illegal code should be regarded as unpredictable and unreliable.


So you're claiming all accepts-invalid bugs are really enhancement requests?

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bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
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                 CC|                            |bearophile_hugs eml.cc


--- Comment #6 from bearophile_hugs eml.cc 2011-10-26 15:10:50 PDT ---
(In reply to comment #5)


 Moreover, why does the spec bother to forbid anything if compiler writers are
 free to ignore it all?


Specs are meant to be long-term things. When you write the specs for a language
as complex as D you are free to write things that your current working compiler
is not yet able to do, or you have not yet implemented. There are several
things written in D specs (or TDPL) that DMD is not yet doing.

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