[jwalden@find-waldo-now tmp]$ cat amb.cpp
#include <iostream>

template<typename derived_t>
struct protocol_object {};

struct data_object : public protocol_object<data_object> {};

struct conversion_required { conversion_required(int) {} };

template<typename derived_t>
void match_impl(protocol_object<derived_t> &value, int)
{
   std::cout << "protocol_class";
};

template<typename T>
void match_impl(T &value, conversion_required)
{
   std::cout << "any other type";
};

template<typename T>
void match(T& value)
{
    return match_impl(value, 0);
}

int main()
{
  data_object d;
  int i;
  match(d);
  match(i);
}
[jwalden@find-waldo-now tmp]$ g++48 -oamb amb.cpp
amb.cpp: In instantiation of ‘void match(T&) [with T = data_object]’:
amb.cpp:32:10:   required from here
amb.cpp:25:31: warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second: [enabled by default]
     return match_impl(value, 0);
                               ^
amb.cpp:11:6: note: candidate 1: void match_impl(protocol_object<derived_t>&, int) [with derived_t = data_object]
 void match_impl(protocol_object<derived_t> &value, int)
      ^
amb.cpp:17:6: note: candidate 2: void match_impl(T&, conversion_required) [with T = data_object]
 void match_impl(T &value, conversion_required)
      ^
[jwalden@find-waldo-now tmp]$ clang++-tip -oamb amb.cpp
amb.cpp:25:12: error: call to 'match_impl' is ambiguous
    return match_impl(value, 0);
           ^~~~~~~~~~
amb.cpp:32:3: note: in instantiation of function template specialization 'match<data_object>' requested here
  match(d);
  ^
amb.cpp:11:6: note: candidate function [with derived_t = data_object]
void match_impl(protocol_object<derived_t> &value, int)
     ^
amb.cpp:17:6: note: candidate function [with T = data_object]
void match_impl(T &value, conversion_required)
     ^
1 error generated.