bitset compile error

#include <iostream>

using namespace std;

template<unsigned int NBITS>
struct bitset {
    static constexpr unsigned int SIZE = NBITS;
    static constexpr unsigned int NWORDS = (NBITS & 63) ? ((NBITS>>6)+1) : (NBITS>>6);
    unsigned long words[NWORDS];
    unsigned int num_in_set;

    bitset() : num_in_set(0) {
        memset(words, 0, sizeof(words));
    }

    bool has(int ix) const {
        return (words[ix>>6] >> (ix & 63)) & 1;
    }
    void set(int ix) {
        unsigned long& w(words[ix>>6]);
        unsigned long b = 1UL << (ix & 63);
        if (!(w & b)) {
            w |= b;
            num_in_set++;
        }
    }
    void clr(int ix) {
        unsigned long& w(words[ix>>6]);
        unsigned long b = 1UL << (ix & 63);
        if (w & b) {
            w &= ~b;
            num_in_set--;
        }
    }
};


typedef bitset<32> bitset_combos;

int main()
{
    bitset_combos b;
    b.set(1);
    cout << b.num_in_set << endl;

    return 0;
}



$ clang++ -std=c++11 minimal.cpp
minimal.cpp:39:9: error: unknown type name 'bitset'; did you mean '__bitset'?
typedef bitset<32> bitset_combos;
        ^~~~~~
        __bitset
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/bitset:146:7: note: '__bitset' declared here
class __bitset
      ^
minimal.cpp:39:15: error: expected unqualified-id
typedef bitset<32> bitset_combos;
              ^
minimal.cpp:43:5: error: unknown type name 'bitset_combos'
    bitset_combos b;
    ^
3 errors generated.