[Bug c++/19404] [4.0 Regression] anonymous types and templates and rejecting valid code

["lars at trolltech dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From lars at trolltech dot com  2005-04-08 16:35 -------
Subject: Re:  [4.0 Regression] anonymous types and templates and rejecting valid code

On Friday 08 April 2005 17:34, mark at codesourcery dot com wrote:
> ------- Additional Comments From mark at codesourcery dot com  2005-04-08
> 15:34 ------- Subject: Re:  [4.0 Regression] anonymous types and templates
>  and rejecting valid code
>
> lars at trolltech dot com wrote:
> > ------- Additional Comments From lars at trolltech dot com  2005-04-08
> > 14:36 ------- Hi Mark,
> >
> > (In reply to comment #3)
> >
> >>This code is invalid.  The use of ">>" requires the instantiation of the
> >>declaration of the overloaded "operator>>", but that instantiation fails
> >
> > because
> >
> >>one of the template argument is anonymous.
> >
> > I don't see why the code is invalid. If you leave out the forward
> > declaration of the template operator everything compiles fine, as the
> > values in the anonymous enum get cast to integers, and the builtin
> > operator>>(int, int) applies.
>
> Lars --
>
> As I said earlier in the thread, there is now an open DR about this
> issue.  So, it might be resolved either way, eventually.

Sorry, I didn't see the connection clearly. I was just wondering as we got a 
few bug reports about compile failures using Qt with gcc 4.

> But, what the standard says at present is that you do overload
> resolution on both operators.  That requires that you instantiate their
> declarations, and the instantiation of the one using anonymous enums is
> invalid.  There's nothing in the standard that says that you discard
> instantiations that don't work -- except in cases of SFINAE.  And SFINAE
> does not let you ignore all errors, as some people think; it's a very
> specific set.

I haven't read the standard to that level I have to admit :)

But if this invalid instantiation can't be ignored it seems to mean to me that 
one can not use any anonymous enum in C++ (better in a C++ header), as you 
would get compile errors as soon as someone uses a template to overload an 
operator that happens to be used together with values in the enum.

Cheers,
Lars


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19404