Bug? std::string promoted to bool???

[U.Mutlu]

Why does this compile?
The 2nd par should be a bool not a std::string.
IMO the compiler should generate a warning (actually even an error) about it.

#include <string>

struct TS {};

bool func(const TS& S, const bool f, const std::string s = "")
  {
    //...
    return f;
  }

int main()
  {
    TS S;
    func(S, "X");   // <-- HERE
    return 0;
  }


--
Thx
Uenal