Fix compilation issues

Author: ypatia

tiledb/common/thread_pool/thread_pool.h

@@ -52,9 +52,13 @@ class ThreadPool {
    */
   class ThreadPoolTask {
    public:
-    ThreadPoolTask() = default;
+    ThreadPoolTask() = delete;
     ThreadPoolTask(ThreadPool* tp)
-        : tp_(tp){};
+        : tp_(tp) {
+      if (tp == nullptr) {
+        throw std::runtime_error("Cannot construct task for a null threadpool");
+      }
+    };
     virtual ~ThreadPoolTask(){};
 
    protected:
@@ -79,11 +83,15 @@ class ThreadPool {
    */
   class Task : public ThreadPoolTask {
    public:
+    Task() = delete;
+
     /**
-     * Default constructor
-     * @brief Default constructed SharedTask is possible but not valid().
+     * Constructor
+     * @brief Initializes only the threadpool but the future it encapsulates is
+     * not valid().
      */
-    Task() = default;
+    Task(ThreadPool* tp)
+        : ThreadPoolTask(tp){};
 
     /**
      * Constructor from std::future
@@ -165,11 +173,15 @@ class ThreadPool {
    */
   class SharedTask : public ThreadPoolTask {
    public:
+    SharedTask() = delete;
+
     /**
-     * Default constructor
-     * @brief Default constructed SharedTask is possible but not valid().
+     * Constructor
+     * @brief Initializes only the threadpool but the shared_future it
+     * encapsulates is not valid().
      */
-    SharedTask() = default;
+    SharedTask(ThreadPool* tp)
+        : ThreadPoolTask(tp){};
 
     /**
      * Constructor from std::shared_future
@@ -318,7 +330,7 @@ class ThreadPool {
   template <class Fn, class... Args>
   auto async(Fn&& f, Args&&... args) {
     if (concurrency_level_ == 0) {
-      Task invalid_future;
+      Task invalid_future(this);
       LOG_ERROR("Cannot execute task; thread pool uninitialized.");
       return invalid_future;
     }

tiledb/sm/query/readers/dense_reader.cc

@@ -368,7 +368,7 @@ Status DenseReader::dense_read() {
   // This is as far as we should go before implementing this properly in a task
   // graph, where the start and end of every piece of work can clearly be
   // identified.
-  ThreadPool::SharedTask compute_task;
+  ThreadPool::SharedTask compute_task(&resources_.compute_tp());
 
   // Allow to disable the parallel read/compute in case the memory budget
   // doesn't allow it.