[PATCH] Instantiate static constexpr member function of a local struct in a function template earlier.

[Richard Smith]

On Tue, Apr 28, 2015 at 12:14 PM, Michael Park <mcypark at gmail.com> wrote:

> Thanks for bringing this up Richard!
>
> we reject the corresponding non-template case
>>
>
> It looks like we *correctly* reject the non-template case.
>

Right.


> The GCC folks ran into that case and declared it invalid:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51080.
>
> but accept the corresponding non-local class template case.
>>
>
> In terms of standardese, I haven't dug deep enough to determine whether
> this case is valid or not. Perhaps you know the answer to this already?
>

The answer is that we don't really know. It would be more consistent with
the non-template case for it to be ill-formed (it also seems reasonable to
say that you can't instantiate a member function until its class is
complete), but the point of instantiation rules for constexpr functions
aren't settled yet; this is part of DR1581 (still open).

In terms of our implementation, given that the non-template case is
> ill-formed:
>
> struct X {
>>     static constexpr int get() { return 42; }
>
>     int arr[get()];  // ill-formed: get() not defined here.
>> };  // get() is defined here.
>
>
> It makes sense to me that the following would not work either:
>
> void f() {
>
>   struct X {
>>     static constexpr int get() { return 42; }
>>     int arr[get()];
>>   }
>> }
>
>
> Given that the class template case works (again, not sure about the
> standardese):
>
>
>> template <typename>
>
> struct X {
>
>     static constexpr int get() { return 42; }
>
>     int arr[get()];
>
> };
>
>
>
> template class X<void>;
>
>
> I think the following should work also:
>
> template <typename>
>
> void f() {
>
>   struct X {
>>     static constexpr int get() { return 42; }
>>     int arr[get()];
>>   }
>> }
>
>
> I think we need to distinguish the 2 cases (1) a local class enclosed in a
> non-template function and (2) a local class enclosed in a function
> template. A local class within a non-template function would behave
> similarly to a non-template class whereas a local class within a function
> template would behave similarly to a class template. Currently we seem to
> treat a local class similar to a non-template class whether it's enclosed
> in a non-template function or a function template.
>
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