Is this use of the keyword struct invalid?

Hi all,

I bumped into a compilation error with GCC 4.8 with the following code:

$ cat t.cc
class B
{
protected:
  struct S {};
  virtual void doIt(struct S& s) = 0;
};

class D : public B
{
public:
  using B::S;
  virtual void doIt(struct S& s);
};

void D::doIt(struct S& s)
{
  (void)s;
}

$ g++ -Wall -Wextra -Woverloaded-virtual -c t.cc
t.cc:5:16: warning: 'virtual void B::doIt(B::S&)' was hidden
[-Woverloaded-virtual]
   virtual void doIt(struct S& s) = 0;
                ^
t.cc:12:16: warning: by 'virtual void D::doIt(S&)' [-Woverloaded-virtual]
   virtual void doIt(struct S& s);
                ^
t.cc:15:6: error: prototype for 'void D::doIt(B::S&)' does not match any in
class 'D'
 void D::doIt(struct S& s)
      ^
t.cc:12:16: error: candidate is: virtual void D::doIt(S&)
   virtual void doIt(struct S& s);
                ^

When I remove struct in the declaration of doIt(), everything compiles fine.
What's going on here?

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