[Bug c++/53184] Unnecessary anonymous namespace warnings

["zeratul976 at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53184

--- Comment #6 from Nathan Ridge <zeratul976 at hotmail dot com> ---
(In reply to Jason Merrill from comment #4)
> (In reply to comment #3)
> > Because then the anonymous class has the name "Foo" for linkage purposes, and
> > has external linkage.  When Foo referes to the const or volatile qualified form
> > of the class, the anonymous class itself has no name and so no linkage, only
> > the cv-qualified form has a name for linkage purposes.
> > 
> > I'm not sure if that behaviour is correct though, let's ask Jason
> 
> Yes, that's right. 7.1.3:
> 
> If the typedef declaration defines an unnamed class (or enum), the first
> typedef-name declared by the declaration to be that class type (or enum
> type) is used to denote the class type (or enum type) for linkage purposes
> only (3.5).
> 
>   typedef struct { } *ps, S;  // S is the class name for linkage purposes
> 
> Adding volatile means that Foo doesn't name the class, it names the volatile
> variant of the class, so it isn't the class's name for linkage purposes, so
> the class has no linkage.  The warning complains about using a type with no
> linkage as a member type in a header file, since including that header in
> multiple source files would be an ODR violation as the type Foo is different
> in each translation unit.

Is this ODR violation an actual problem in practice, with GCC? I find these
warnings annoying to work around, and I wouldn't mind seeing an option to
silence them if they don't indicate an actual problem.