[cfe-dev] Typeid and forward declaration - Clarification needed

[Richard Smith]

On Tue, Oct 29, 2013 at 2:40 PM, Karen Shaeffer <shaeffer at neuralscape.com>wrote:

> On Tue, Oct 29, 2013 at 02:14:11PM -0700, Richard Smith wrote:
> > On Tue, Oct 29, 2013 at 1:29 AM, Rahul Jain <1989.rahuljain at gmail.com
> >wrote:
> >
> > >
> > > Hi all,
> > >
> > > This is with respect to the following TC.
> > >
> > > struct S;
> > >
> > > template <typename T>
> > > static bool fn (S *s)
> > > {
> > >   return typeid (*s) == typeid (T);
> > > }
> > >
> > > struct S
> > > {
> > > };
> > >
> > > bool x = fn<S> (__null);
> > >
> > >
> > > The standard says:
> > > If the type of the type-id is a class type or a reference to a class
> type,
> > > the class shall be completely-defined.
> > >
> > >
> > > This "shall" is kind of confusing. Is this a valid TC. Is forward
> > > declaration allowed while using the typeid operator?
> > >
> >
> > No, but the rules are slightly different because this occurs inside a
> > template. This testcase is ill-formed, but no diagnostic is required, per
> > [temp.res](14.6)p8:
> >
> > "If a type used in a non-dependent name is incomplete at the point at
> which
> > a template is defined but is complete at the point at which an
> > instantiation is done, and if the completeness of that type affects
> whether
> > or not the program is well-formed or affects the semantics of the
> program,
> > the program is ill-formed; no diagnostic is required."
> >
> > So the template has no valid specializations. Further, we're allowed to
> > reject this even if the template is never instantiated:
> >
> > "If no valid specialization can be generated for a template, and that
> > template is not instantiated, the template is ill-formed, no diagnostic
> > required."
> >
>
> Hi Richard,
> There is no ambiguity there. The standard defines it concisely. I actually
> thought
> whether a compiler rejected that example was implementation specific,
> depending on
> the compiler's template instantiation stategy. But c++11 defines it as an
> ill-formed
> program. Is that test case an ill-formed program in the previous standard?
>

The wording was added by DR206 in response to the ambiguity in the original
(C++98) spec. Since this was a DR, it retroactively applies to C++98 too,
so this code has (in some sense) always been ill-formed, with no diagnostic
required:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#206

I'm not sure what you're talking about when you mention ambiguity.
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