Why do I get: cannot declare variable ‘sc’ to be of abstract type...

[David Daney <ddaney at avtrex dot com>]

When I compile this:
------foo.cc----------
class I1
{
public:
   virtual void a() = 0;
};

class C1 : public I1
{
public:
   void a()
   {
   }
};

class I2 : public I1
{

public:
   virtual void b() = 0;
};

class C2 : public I2, public C1
{
public:
   void b()
   {
   }
};

int main(int argc, char *argv[])
{
   C2 sc;
   return 0;
}
---------------------
$ g++ -c -g foo.ccfoo.cc: In function ‘int main(int, char**)’:
foo.cc:33: error: cannot declare variable ‘sc’ to be of abstract type ‘C2’
foo.cc:24: note:   because the following virtual functions are pure 
within ‘C2’:
foo.cc:5: note:         virtual void I1::a()

I would think that I1::a() is not virtual because it is defined in the 
super class C1.  What is going on here?

$ g++ --version
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)

Same results for:$ ~/gccsvn/native-clean/gcc/xgcc 
-B/home/daney/gccsvn/native-clean/gcc/ --version
xgcc (GCC) 4.3.0 20070728 (experimental)

David Daney