[cfe-dev] Endless operator-> chain causing infinite loop

[Rahul Jain]

Hi Richard / Karen,

Thanks for making things clearer.
Now how do we go about handling this sort of behavior?

Do we throw an error message just like g++ does? Wont it be better to print
an error message instead of an infinite loop?

Also if you could please elaborate a bit on how we can "limit the number of
times we go around the loop looking for an overloaded operator-> in Sema::
ActOnStartCXXMemberReference" ?

Thanks,
Rahul




On Wed, Oct 23, 2013 at 3:30 AM, Karen Shaeffer <shaeffer at neuralscape.com>wrote:

> On Tue, Oct 22, 2013 at 02:54:08PM -0700, Richard Smith wrote:
> > On Tue, Oct 22, 2013 at 2:03 PM, Karen Shaeffer <
> shaeffer at neuralscape.com>wrote:
> >
> > > On Tue, Oct 22, 2013 at 12:14:25PM -0700, Richard Smith wrote:
> > > > On Tue, Oct 22, 2013 at 2:45 AM, Rahul Jain <
> 1989.rahuljain at gmail.com
> > > >wrote:
> > > >
> > > > >
> > > > > Hi all,
> > > > >
> > > > > clang version 3.4 (192772)
> > > > >
> > > > > This is with respect to the following gcc testsuite TC:
> > > > >
> > > > > template< int n >
> > > > > struct a {
> > > > >     a< n+1 > operator->()
> > > > >         {
> > > > >         return a< n+1 >();
> > > > >         }
> > > > > };
> > > > >
> > > > > int main() {
> > > > >     a<0>()->x;
> > > > > }
> > > > >
> > > > >
> > > > > This TC goes into an infinite loop when compiled. Ideally it should
> > > throw
> > > > > the error
> > > > > recursive template instantiation exceeded maximum depth of 256.
> > > > >
> > > >
> > > > That's not the right behavior; there's no recursive template
> > > instantiation
> > > > here. Each operator-> is instantiated from within the context of
> 'main',
> > > > not from within some other instantiation.
> > > >
> > > > If we want to limit this, we should put a limit on the number of
> times we
> > > > go around the loop looking for an overloaded operator->
> > > > in Sema::ActOnStartCXXMemberReference. However, I've seen people use
> this
> > > > in practice in template metaprogramming to get around the recursive
> > > > template instantiation depth, so this might break existing code.
> > >
> > > Hi Richard,
> > >
> > > Just checked, and g++-4.7.2 doesn't give a 'recusive template
> > > instantiation' error.
> > > The error is:
> > >
> > > error: template instantiation depth exceeds maximum of 900 (use
> > > -ftemplate-depth= to increase the maximum) instantiating ‘a<(n + 1)>
> > > a<n>::operator->() [with int n = 900]’
> > >
> >
> > This is their recursive template instantiation error. I'm not sure why
> they
> > chose to diagnose this problem in that way; there is no recursive
> > instantiation here. The loop is in [over.ref](13.5.6)p1:
> >
> > "An expression x->m is interpreted as (x.operator->())->m for a class
> > object x of type T if T::operator->() exists and if the operator is
> > selected as the best match function by the overload resolution
> mechanism."
> >
> > This rule gets applied repeatedly, outside of any template instantiation.
> >
> >
> > > In the standard at [temp.inst]
> > >
> >
> > > <14.7.1 note d>
> > > There is an implementation-defined quantity that specifies the limit on
> > > the total depth
> > > of recursive instantiations, which could involve more than one
> template.
> > > The result of an
> > > infinite recursion in instantiation is undefined.
> > >
> >
> > This is not what's happening in this case.
>
> Hi Richard,
> I understand it is not what is happening. I just included the <14.7.2 note
> 15>
> for clarity. I think the g++ error message is crystal clear. And the
> practicality
> of it is obvious.
>
> Thanks for your comments.
>
> enjoy,
> Karen
> --
> Karen Shaeffer                 Be aware: If you see an obstacle in your
> path,
> Neuralscape Services           that obstacle is your path.        Zen
> proverb
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20131023/086c573b/attachment.html>