C++重载函数问题

为什么编译器找不到基类函数签名?改变 foo( a1 ) 到 B::foo( a1 ) 作品。

代码:

class A1 ;
class A2 ;

class B
{
public:
   void foo( A1* a1 ) { a1 = 0 ; }
} ;

class C : public B
{
public:
   void foo( A2* /*a2*/ )
   {
      A1* a1 = 0 ;
      foo( a1 ) ;
   }
} ;

int main()
{
   A2* a2 = 0 ;
   C c ;
   c.foo( a2 ) ;
   return 0 ;
}

编译器错误(VS2008):

error C2664: 'C::foo' : cannot convert parameter 1 from 'A1 *' to 'A2 *'
Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast