[Bug c++/54055] New: spurious(?) "invalid use of incomplete type" warning in template definition

["roland at gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54055

             Bug #: 54055
           Summary: spurious(?) "invalid use of incomplete type" warning
                    in template definition
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: roland@gnu.org


Created attachment 27847
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27847
test case, C++ source

The attached input file is culled from Chromium sources.  Past versions of
GCC do not complain about this and nor does Clang.  Trunk G++ started
giving a warning fairly recently (sometime after I left for Prague).
It happens with no special options, and also with -std=gnu++98; I didn't
test other modes.

foo.cc:3:35: warning: invalid use of incomplete type ‘class scoped_ptr<C>’
[enabled by default]
   private: struct RValue : public scoped_ptr { RValue(); ~RValue();
RValue(cons
                                   ^
foo.cc:2:7: warning: declaration of ‘class scoped_ptr<C>’ [enabled by default]
 class scoped_ptr {
       ^

This code pattern comes from using the macro MOVE_ONLY_TYPE_FOR_CPP_03
as defined here:
    http://src.chromium.org/viewvc/chrome/trunk/src/base/move.h?view=markup

Is this code really invalid C++?  If so, was it intended to start rejecting
it without any new -std= or -Wfoo options when G++ never did before?