Re: Bug in c++11 mode when instantiating template with local class

[Edward Diener <eldlistmailingz at tropicsoft dot com>]

On 6/24/2017 7:01 AM, Jonathan Wakely wrote:
> On 23 June 2017 at 18:43, Edward Diener wrote:
> > On 6/23/2017 10:46 AM, Jonathan Wakely wrote:
> > > I don't think it's a bug. The code can be simplified considerably:
> > >
> > > template<void(*)()> struct helper { };
> > >
> > > struct AClass
> > > {
> > >     static void StaticFunction() { }
> > > };
> > >
> > > int main()
> > > {
> > >     struct ANestedClass
> > >     {
> > >       static void StaticFunction() { }
> > >     };
> > >
> > >     helper<&AClass::StaticFunction> ok;
> > >     helper<&ANestedClass::StaticFunction> error;
> > > }
> > >
> > > GCC, Clang and EDG all reject this for the same reason: you're using
> > > the address of a function with no linkage for a non-type template
> > > argument.
> > >
> > > [basic.link] says that although ANestedClass has internal linkage, the
> > > static member function has no linkage.
> >
> >
> > Could you point out where in [basic.link] this is specified ? I admit I have
> > a great deal of trouble understanding the rules for internal, external, and
> > no linkage of names in section 3.5 of the C++ standard. But I do find it odd
> > that if a local class name has internal linkage, a static member function
> > name of that local class has no linkage.
>
> I misread it as saying ANestedClass has internal linkage, I think it
> also has no linkage (which would be why its member has no linkage).
>
> Parargaph 8: "Names not covered by these rules have no linkage."

You are correct.

> > > N.B. https://wg21.link/n2187 allowed types with internal linkage to be
> > > used for template arguments, and https://wg21.link/cwg1155 allowed the
> > > addresses of objects with internal linkage to be used for non-type
> > > template arguments, but internal linkage is not the same as no
> > > linkage.
> > >
> > > I believe the reason the code is invalid is because otherwise you
> > > could declare an object with external linkage using a type with no
> > > linkage:
> > >
> > > static helper<&ANestedClass::StaticFunction> foo;
> >
> >
> > In your example doesn't 'foo' have internal linkage because it is declared
> > 'static' ?
>
> No, that only applies to static variables at namespace-scope. Looking
> at it again, that name has no linkage either (it can't be referred to
> from other scopes). So I'm not sure of the reason for the rule against
> using types with no linkage as non-type template arguments.

I asked about this at
https://groups.google.com/a/isocpp.org/forum/#!forum/std-discussion
with the question
https://groups.google.com/a/isocpp.org/forum/#!topic/std-discussion/f4Tmi6vibuQ.
So far I have not received any answers. If you know of any other place 
where I might receive an explanation please say so. I understand that 
gcc is doing the correct thing, even if I do not understand the C++ 
standard reason for the rule.