Why is there ambiguity between uint32_t and uint64_t when using size_t on Mac OS X?

Consider following example code:

#include <iostream>
#include <inttypes.h>

using namespace std;

int f(uint32_t i)
{
  return 1;
}
int f(uint64_t i)
{
  return 2;
}

int main ()
{
  cout << sizeof(long unsigned) << '\n';
  cout << sizeof(size_t) << '\n';
  cout << sizeof(uint32_t) << '\n';
  cout << sizeof(uint64_t) << '\n';
  //long unsigned x = 3;
  size_t x = 3;
  cout << f(x) << '\n';
  return 0;
}

This fails on Mac OSX with:

$ g++ --version
i686-apple-darwin10-g++-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)
$ make test
g++     test.cc   -o test
test.cc: In function 'int main()':
test.cc:23: error: call of overloaded 'f(size_t&)' is ambiguous
test.cc:6: note: candidates are: int f(uint32_t)
test.cc:10: note:                 int f(uint64_t)
make: *** [test] Error 1

Why? Because 'size_t' should be unsigned and either 32 bit or 64 bit wide. Where is the ambiguity then?

Trying the same with 'unsigned long x' instead of 'size_t x' results in an analogous ambiguity error message.

On Linux/Solaris systems, testing with different GCC versions, different architectures etc. there is no ambiguity reported (and the right overload is used on each architecture).

Is this a Mac OS X bug or a feature?