Important unit tests for your shared_ptr implementation:

gistfile1.txt

- Self assignment: May accidentally destroy the shared_block in the assignment
      shared<X> a = make_shared<X>();
      a = a;

- Weak pointer in child: Depending on the implementation, could delete the control block twice.
    struct Test {
      weak<Test> self;
    };
    shared<Test> t = make_shared<Test>();
    t->self = t;
    t.reset();
    
- Feel free to comment if you find other difficult to find things that can go wrong!