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

[Karen Shaeffer]

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