[Bug c++/31954] New: template function cannot create object of forward declared class

["thomas at maier-komor dot de" <gcc-bugzilla at gcc dot gnu dot org>]

The following code works on gcc-4.1.2, but is rejected with gcc-4.2.0:

class ABC0
{
        public:
        virtual ~ABC0() = 0;
        virtual int something() = 0;
};

inline ABC0::~ABC0()
{ }

struct C0;

struct ABC1 : public ABC0
{
        explicit ABC1()
        : ABC0()
        { }

        virtual ABC0 *sometwhat() = 0;
};


template<class X>
struct C1 : public ABC1
{
        ABC0 *somewhat();
};


template<class X>
ABC0 *C1<X>::somewhat()
{
        return new C0;
}


struct C0 : public ABC0
{
        C0(int c)
        : ABC0()
        , _c(c)
        { }

        int something()
        { return 0; }

        private:
        int _c;
};


$ g++ -V4.2.0 -c bug.cc
bug.cc: In member function 'ABC0* C1<X>::somewhat()':
bug.cc:34: error: invalid use of incomplete type 'struct C0'
bug.cc:12: error: forward declaration of 'struct C0'


-- 
           Summary: template function cannot create object of forward
                    declared class
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: thomas at maier-komor dot de
 GCC build triplet: i686-pc-cygwin
  GCC host triplet: i686-pc-cygwin
GCC target triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31954