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."
>>
>> 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.
