U>#include <memory>
U>int main()
U>{
U>  std::unique_ptr<int> a;
U>  bool x = a;
U>}
U>

U>в VS2010 код компилируется

typename add_lvalue_reference<T>::type operator*() const;
pointer operator->() const noexcept;
pointer get() const noexcept;
deleter_type& get_deleter() noexcept;
const deleter_type& get_deleter() const noexcept;
explicit operator bool() const noexcept;

2 A conversion function may be explicit (7.1.2), in which case it is only considered as a user-defined conversion
for direct-initialization (8.5). Otherwise, user-defined conversions are not restricted to use in assignments
and initializations. [ Example:

class Y { };

struct Z {
  explicit operator Y() const;
};

void h(Z z) {
  Y y1(z); // OK: direct-initialization
  Y y2 = z; // ill-formed: copy-initialization
  Y y3 = (Y)z; // OK: cast notation
}