COMP: Let Future.get() calls in PoolMultiThreader always return void

`Future` was declared as `std::future<ITK_THREAD_RETURN_TYPE>`, which is
platform specific, and caused warnings on some platforms, as reported by Hans
Johnson at InsightSoftwareConsortium#5945 :

    itkPoolMultiThreader.cxx:148:55: warning:
      ignoring return value of function declared with 'nodiscard'
      attribute [-Wunused-result]
        exceptionHandler.TryAndCatch([this, threadLoop] { m_ThreadInfoArray[threadLoop].Future.get(); });
                                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Addressed by replacing the public `ThreadPoolInfoStruct` with a private
`InternalWorkUnitInfo` struct, which has its `Future` declared as
`std::future<void>`.

Author: N-Dekker

Modules/Core/Common/include/itkPoolMultiThreader.h

@@ -95,10 +95,14 @@ class ITKCommon_EXPORT PoolMultiThreader : public MultiThreaderBase
   void
   SetMaximumNumberOfThreads(ThreadIdType numberOfThreads) override;
 
-  struct ThreadPoolInfoStruct : WorkUnitInfo
+#ifndef ITK_FUTURE_LEGACY_REMOVE
+  struct ITK_FUTURE_DEPRECATED(
+    "PoolMultiThreader now uses a slightly different private `InternalWorkUnitInfo` struct instead!")
+    ThreadPoolInfoStruct : WorkUnitInfo
   {
     std::future<ITK_THREAD_RETURN_TYPE> Future;
   };
+#endif
 
 protected:
   PoolMultiThreader();
@@ -107,13 +111,18 @@ class ITKCommon_EXPORT PoolMultiThreader : public MultiThreaderBase
   PrintSelf(std::ostream & os, Indent indent) const override;
 
 private:
+  struct InternalWorkUnitInfo : WorkUnitInfo
+  {
+    std::future<void> Future;
+  };
+
   // Thread pool instance and factory
   ThreadPool::Pointer m_ThreadPool{};
 
   /** An array of work unit information containing a work unit id
    *  (0, 1, 2, .. ITK_MAX_THREADS-1), work unit count, and a pointer
    *  to void so that user data can be passed to each thread. */
-  ThreadPoolInfoStruct m_ThreadInfoArray[ITK_MAX_THREADS]{};
+  InternalWorkUnitInfo m_ThreadInfoArray[ITK_MAX_THREADS]{};
 
   /** Friends of Multithreader.
    * ProcessObject is a friend so that it can call PrintSelf() on its

Modules/Core/Common/src/itkPoolMultiThreader.cxx

@@ -132,7 +132,8 @@ PoolMultiThreader::SingleMethodExecute()
   {
     m_ThreadInfoArray[threadLoop].UserData = m_SingleData;
     m_ThreadInfoArray[threadLoop].NumberOfWorkUnits = m_NumberOfWorkUnits;
-    m_ThreadInfoArray[threadLoop].Future = m_ThreadPool->AddWork(m_SingleMethod, &m_ThreadInfoArray[threadLoop]);
+    m_ThreadInfoArray[threadLoop].Future = m_ThreadPool->AddWork(
+      [method = m_SingleMethod, threadInfo = &m_ThreadInfoArray[threadLoop]] { method(threadInfo); });
   }
 
   // Now, the parent thread calls this->SingleMethod() itself
@@ -175,8 +176,6 @@ PoolMultiThreader::ParallelizeArray(SizeValueType             firstIndex,
       {
         aFunc(ii);
       }
-      // make this lambda have the same signature as m_SingleMethod
-      return ITK_THREAD_RETURN_DEFAULT_VALUE;
     };
 
     SizeValueType workUnit = 1;
@@ -265,11 +264,8 @@ PoolMultiThreader::ParallelizeImageRegion(unsigned int         dimension,
         total = splitter->GetSplit(i, splitCount, iRegion);
         if (i < total)
         {
-          m_ThreadInfoArray[i].Future = m_ThreadPool->AddWork([funcP, iRegion]() {
-            funcP(&iRegion.GetIndex()[0], &iRegion.GetSize()[0]);
-            // make this lambda have the same signature as m_SingleMethod
-            return ITK_THREAD_RETURN_DEFAULT_VALUE;
-          });
+          m_ThreadInfoArray[i].Future =
+            m_ThreadPool->AddWork([funcP, iRegion]() { funcP(&iRegion.GetIndex()[0], &iRegion.GetSize()[0]); });
         }
         else
         {