void M_relocate(T* newB) {
    relocate_move(newB, impl_.b_, impl_.e_);
    relocate_done(newB, impl_.b_, impl_.e_);
  }

  void relocate_move_or_memcpy(T* dest, T* first, T* last, std::true_type) {
    if (first != nullptr) {
      std::memcpy((void*)dest, (void*)first, (last - first) * sizeof(T));
    }
  }

  void relocate_done(T* /*dest*/, T* first, T* last) noexcept {
    if (folly::IsRelocatable<T>::value && usingStdAllocator) {
      // used memcpy; data has been relocated, do not call destructor
    } else {
      D_destroy_range_a(first, last);
    }
  }


^^^^^^^^^^^^^ If `T = std::unique_pointer<int>`, how will the original objects know to not deallocate the `int`s when they go out of scope?

(Code taken from: https://github.com/facebook/folly/blob/5445a6d9761fb511cbd0add6e31b84908b387948/folly/FBVector.h#L658)