[Bug c++/51440] New: C++ compiler produces warning for an unnamed struct member: TYPE has a field FIELD whose type uses the anonymous namespace

["blue.dragan+gcc at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51440

             Bug #: 51440
           Summary: C++ compiler produces warning for an unnamed struct
                    member: TYPE has a field FIELD whose type uses the
                    anonymous namespace
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: blue.dragan+gcc@gmail.com


Repro steps:

File test.h:
========================================
typedef volatile struct {
} Unnamed;

typedef struct __Named {
    Unnamed u;
} Named;
========================================

File test.cpp:
========================================
#include "test.h"
========================================

Outcome:
========================================
$ g++ -c -o test.o test.cpp
In file included from test.cpp:1:0:
test.h:4:16: warning: '__Named' has a field '__Named::u' whose type uses the
anonymous namespace [enabled by default]
========================================

Details about the repro scenario:

1) Only C++ compiler (C compiler does not generate the warning).

2) Only if the structures are in header files. When the contents of test.h are
moved to test.cpp, the warning disappears.

3) Only if the unnamed structure is volatile. When volatile keyword is removed,
the warning disappears.

4) GCC 4.6.1 hits this problem, but GCC 4.2.4 and 4.4.3 don't have this
problem. Haven't tested any other version of GCC.

5) I'm using GCC for armv7a-unknown-linux-gnueabi target, haven't tried it with
any other target, although the problem "seems" target-independent.

6) The warning is especially annoying, because there appears to be no way to
disable it.

I don't see anything wrong with defining the structures like this. This is a
common way for defining hardware registers which helps to prevent users from
mistakenly dropping the volatile keyword.