[patch] Fix pr16247

[Richard Smith]

On Mon, Jun 10, 2013 at 4:08 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
> Richard pointed out on IRC that the patch should look at the
> redeclaration context to avoid problems with one of the functions
> being in a extern "C++". Implementing that found some interesting
> problems. Consider
>
> extern "C" {
>   static void foo(int x);
>   static void foo() {
>   }
>   void foo(int x) {
>   }
> }
>
> This should be valid, since both functions have internal linkage and
> therefore none of them has C language linkage. Commenting the first
> declaration makes the code invalid as now the last one is extern C and
> [dcl.link] p6 kicks in. This means we have to do "normal" overload
> resolution first to see if the last decl we are looking at has C
> language linkage or not.
>
> Implementing this without doing two passes over the loopkup result is
> not trivial. The check of two function overloading has 3 possibilities
> * It is a redeclaration
> * it is an overload
> * it is a redeclaration if the last one has C language linkage.
>
> The attached patch implements it, but I really hope there is a better
> way to implement this :-(

Here's another alternative: remove the extern "C" checking from
IsOverload entirely, and use the LocallyScopedExternCDecls mechanism
to enforce the rule instead.