From comment #7 in 29095 we have:

The technical issue is that in the following code:

  extern "C" {
    typedef void (*p1)();
  }
  typedef void (*p2)();

p1 and p2 are distinct types, and, in fact, you can overload based on that. 
G++ doesn't implement that distinction; we don't keep track of language linkage
for types (just for functions) but we should, and, at some point, I'm sure
we'll implement that.  The reason this is in the standard is so that an
implementation can use different calling conventions for C and C++.  So, when
calling through a function pointer you have to know which kind of function
you're calling.  (And, yes, name-mangling is supposed to encode the linkage of
the function type, when mangling a pointer-to-function type.)

End of interesting comment. 

This was news to me, and a quick perusal of "5.1 External Names (a.k.a. Mangling)" in the IA64 C++ spec doesn't find any matches. However, I can see why this might be useful, and am curious as to the behavior of pre-3.0.x GNU compilers, as well as other ABI's (such as microsoft) or EDG in native mode.