force key type of a std::map not to be const

C++ references tells us for a std::map

typedef pair<const Key, T> value_type;

Is it possible to force the Key Type not to be const ? I need to do this in a template method like

template<class T> // T represent a map in general (std::map, boost::unordered_map or whatever..)
void foo(const T& m)
{
  typename T::value_type::first_type x;
  x=0; // Wrong because x is const ...
}