Add further type-safety check to work_stealing_deque.

lib/pls/include/pls/internal/data_structures/work_stealing_deque.h

@@ -85,7 +85,7 @@ class work_stealing_deque {
   std::pair<work_stealing_deque_item, T> *allocate_item(const T &new_item);
 
   template<typename T>
-  Item *push_tail(const T &new_item);
+  T *push_tail(const T &new_item);
   Item *pop_tail();
   Item *pop_head();
 

lib/pls/include/pls/internal/data_structures/work_stealing_deque_impl.h

@@ -38,7 +38,10 @@ std::pair<work_stealing_deque_item, T> *work_stealing_deque<Item>::allocate_item
 
 template<typename Item>
 template<typename T>
-Item *work_stealing_deque<Item>::push_tail(const T &new_item) {
+T *work_stealing_deque<Item>::push_tail(const T &new_item) {
+  static_assert(std::is_same<Item, T>::value || std::is_base_of<Item, T>::value,
+                "Must only push types of <Item> onto work_stealing_deque<Item>");
+
   offset_t local_tail = tail_;
 
   auto new_pair = allocate_item(new_item);