What does the C++ standard have to say about overloading/implementing a templatized version of std::to_string like so:

#include <string>
#include <sstream>
#include <ostream>

namespace std {
  template<typename T>
  string to_string(const T& in)
  {
     ostringstream oss;
     oss << in;
     return oss.str();
  }
}

Is this expressedly disallowed? Or cause nasal-daemons to fly out of the nose?

g++ accepts the above code with --std=c++11 -Wextra -pedantic-errors, reports no errors, and the executable generates the output that I expect (which, of course, does not mean anything in context to my question)