#include <map>
#include <memory>

using namespace std;

int main(void) {
  map<int, unique_ptr<int>> m1;
  map<int, unique_ptr<int>> m2(move(m1));
}

fails to compile under 4.9 (but works fine with 4.7 and 4.8).



A change was introduced in 4.9 after which the move constructor of _Rb_tree invokes _M_copy on a range of elements (which might not be copy-constructible).

Cursory analysis suggests the problem is at /usr/include/c++/4.9/bits/stl_tree.h line 1021.