Patch for Friend function linkage check (PR 15841)

[Richard Smith]

On Wed, Apr 24, 2013 at 7:14 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> On 24 April 2013 21:25, Weiming Zhao <weimingz at codeaurora.org> wrote:
> > Hi Rafael,
> >
> >
> >
> > Could you please check if the attached patch fixes the issue
> > (http://llvm.org/bugs/show_bug.cgi?id=15841) correctly?
> >
>
> I don't think this is the correct fix. getLanguageLinkageTemplate
> should follow the structure of [dcl.link]. I think the real problem is
> the context the friend function is getting.
>
> I went looking in the standard, and what I found was [namespace.memdef] p3:
>
> -------------------------------------------
> Every name first declared in a namespace is a member of that
> namespace. If a friend declaration in a non-local class first declares
> a class, function, class template or function template the friend is a
> member of the innermost enclosing namespace.
> ------------------------------------------
>
> If I read that correctly,
>
> extern "C" {
>   class Foo {
>     friend void bar();
>   };
>   void bar() {
>   }
> }
>
> is equivalent to
>
> void bar();
> extern "C" {
>   class Foo {
>     friend void bar();
>   };
>   void bar() {
>   }
> }
>
> So I now think that clang is correct and the code should be rejected.
> Richard, do you agree? Is this a defect?


The governing rule is [dcl.link]p4: "[...] In a linkage-specification, the
specified language linkage applies to [...] function names with external
linkage [...] declared within the linkage-specification."

"declared within" here is a lexical notion, and the friend function is
declared within the linkage-specification.

(Later in that paragraph, we have "A C language linkage is ignored in
determining the language linkage of the names of class members and the
function type of class member functions", but since a friend isn't a
member, that doesn't apply.)
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