Defining a std::map with an undefined comparison function compiles and links, which surprises me

In std::map, c++11, a function (pointer) specifying the key comparison function may be supplied as an optional parameter. While fiddling around with some code that does so, I got some results from the compiler that I didn't expect. I tried both g++ and clang++. As they both agree, I'm guessing the compilers are good, and I'm confused.

If I declare a std::map-type object in which the comparison function template parameter is specified, but then don't actually define the comparison function, the code still compiles and links and I see no warnings.

I compiled with -std=c++11 -O0 -Wall -Wextra

#include <map>

typedef std::map<int, int, bool(*)(int, int)> Objs;

int main() {

#if 0
  // Compiles and links as expected.
  Objs objs{[](int a, int b) {
    return a < b;
  }};
#endif

#if 1
  // FIXME The lambda definition for the key-comparing function is
  // missing; why does this compile and link?
  Objs objs;
#endif

  objs.emplace(99997, 4210124);
  (void)objs;
  return 0;
}