Add workaround for gcc 16 module

include/beman/execution/detail/atomic_intrusive_stack.hpp

@@ -77,14 +77,11 @@ class atomic_intrusive_stack<Next> {
     //!
     //! @return  If the stack is empty, returns an empty stack.
     auto pop_all_and_shutdown() noexcept -> ::beman::execution::detail::intrusive_stack<Next> {
-        auto  stack = ::beman::execution::detail::intrusive_stack<Next>{};
-        void* ptr   = head_.exchange(this);
+        void* ptr = head_.exchange(this);
         if (ptr == this) {
-            return stack;
+            return {};
         }
-        auto item   = static_cast<Item*>(ptr);
-        stack.head_ = item;
-        return stack;
+        return ::beman::execution::detail::intrusive_stack<Next>{static_cast<Item*>(ptr)};
     }
 
   private:

include/beman/execution/detail/intrusive_stack.hpp

@@ -14,16 +14,17 @@ import std;
 
 namespace beman::execution::detail {
 
-template <auto Next>
-class atomic_intrusive_stack;
-
 template <auto Next>
 class intrusive_stack;
 
 //! @brief  This data structure is an intrusive queue that is not thread-safe.
 template <class Item, Item* Item::* Next>
 class intrusive_stack<Next> {
   public:
+    intrusive_stack() = default;
+
+    explicit intrusive_stack(Item* head) noexcept : head_{head} {}
+
     //! @brief  Pushes an item to the queue.
     auto push(Item* item) noexcept -> void { item->*Next = std::exchange(head_, item); }
 
@@ -43,7 +44,6 @@ class intrusive_stack<Next> {
     auto empty() const noexcept -> bool { return !head_; }
 
   private:
-    friend class atomic_intrusive_stack<Next>;
     Item* head_{nullptr};
 };