[patch] fix pr15930

[Rafael Espíndola]

> Well, improper use of italics aside, my view is that that rule does *not*
> define when an entity has external linkage. Instead, that text is describing
> a consequence of the declaration having external linkage (and by "can be
> referred to" it really means "you don't need to block identical entities in
> different translation units from linking together").
>
> I do agree that the standard may not say what we want here, and isn't
> completely clear; I filed core issue 1602 for that a few months ago for
> exactly that. However, it's not been decided by CWG yet, and since (AFAICT)
> it would only affect which diagnostics are mandatory and it would make
> 'linkage' much more expensive to compute, there's a good chance that they'll
> say it's NAD.

I am not sure I follow the "more expensive" argument. Any compiler (as
opposed to a theoretical tool that just checks if a string is valid
c++) will have to compute something like the linkage we compute, no?

> You should also be aware of core issue 1603, which brings the linkage rules
> more into line with how we behave.

Cheers,
Rafael