Update the scan implementation to follow P0571's guidance.

cub/agent/agent_scan.cuh

@@ -100,12 +100,13 @@ struct AgentScan
     //---------------------------------------------------------------------
 
     // The input value type
-    typedef typename std::iterator_traits<InputIteratorT>::value_type InputT;
+    using InputT = typename std::iterator_traits<InputIteratorT>::value_type;
 
-    // The output value type
-    typedef typename If<(Equals<typename std::iterator_traits<OutputIteratorT>::value_type, void>::VALUE),  // OutputT =  (if output iterator's value type is void) ?
-        typename std::iterator_traits<InputIteratorT>::value_type,                                          // ... then the input iterator's value type,
-        typename std::iterator_traits<OutputIteratorT>::value_type>::Type OutputT;                          // ... else the output iterator's value type
+    // The output value type -- used as the intermediate accumulator
+    // Per https://wg21.link/P0571, use InitValueT if provided, otherwise the
+    // input iterator's value type.
+    using OutputT =
+      typename If<Equals<InitValueT, NullType>::VALUE, InputT, InitValueT>::Type;
 
     // Tile status descriptor interface type
     typedef ScanTileState<OutputT> ScanTileStateT;

cub/device/device_scan.cuh

@@ -158,10 +158,9 @@ struct DeviceScan
         // Signed integer type for global offsets
         typedef int OffsetT;
 
-        // The output value type
-        typedef typename If<(Equals<typename std::iterator_traits<OutputIteratorT>::value_type, void>::VALUE),  // OutputT =  (if output iterator's value type is void) ?
-            typename std::iterator_traits<InputIteratorT>::value_type,                                          // ... then the input iterator's value type,
-            typename std::iterator_traits<OutputIteratorT>::value_type>::Type OutputT;                          // ... else the output iterator's value type
+        // The output value type -- used as the intermediate accumulator
+        // Use the input value type per https://wg21.link/P0571
+        typedef typename std::iterator_traits<InputIteratorT>::value_type OutputT;
 
         // Initial value
         OutputT init_value = 0;

cub/device/dispatch/dispatch_scan.cuh

@@ -254,9 +254,10 @@ template <
     typename InitValueT,          ///< The init_value element type for ScanOpT (cub::NullType for inclusive scans)
     typename OffsetT,            ///< Signed integer type for global offsets
     typename SelectedPolicy = DeviceScanPolicy<
-        typename If<(Equals<typename std::iterator_traits<OutputIteratorT>::value_type, void>::VALUE),  // OutputT =  (if output iterator's value type is void) ?
-        typename std::iterator_traits<InputIteratorT>::value_type,                                          // ... then the input iterator's value type,
-        typename std::iterator_traits<OutputIteratorT>::value_type>::Type> >
+      // Accumulator type.
+      typename If<Equals<InitValueT, NullType>::VALUE,
+                  typename std::iterator_traits<InputIteratorT>::value_type,
+                  InitValueT>::Type>>
 struct DispatchScan:
     SelectedPolicy
 {
@@ -269,11 +270,14 @@ struct DispatchScan:
         INIT_KERNEL_THREADS = 128
     };
 
-    // The output value type
-    typedef typename If<(Equals<typename std::iterator_traits<OutputIteratorT>::value_type, void>::VALUE),  // OutputT =  (if output iterator's value type is void) ?
-        typename std::iterator_traits<InputIteratorT>::value_type,                                          // ... then the input iterator's value type,
-        typename std::iterator_traits<OutputIteratorT>::value_type>::Type OutputT;                          // ... else the output iterator's value type
+    // The input value type
+    using InputT = typename std::iterator_traits<InputIteratorT>::value_type;
 
+    // The output value type -- used as the intermediate accumulator
+    // Per https://wg21.link/P0571, use InitValueT if provided, otherwise the
+    // input iterator's value type.
+    using OutputT =
+      typename If<Equals<InitValueT, NullType>::VALUE, InputT, InitValueT>::Type;
 
     void*           d_temp_storage;         ///< [in] %Device-accessible allocation of temporary storage.  When NULL, the required allocation size is written to \p temp_storage_bytes and no work is done.
     size_t&         temp_storage_bytes;     ///< [in,out] Reference to size in bytes of \p d_temp_storage allocation

test/test_device_scan.cu

@@ -544,24 +544,28 @@ void Initialize(
 template <
     typename        InputIteratorT,
     typename        OutputT,
-    typename        ScanOpT>
+    typename        ScanOpT,
+    typename        InitialValueT>
 void Solve(
     InputIteratorT  h_in,
     OutputT         *h_reference,
     int             num_items,
     ScanOpT         scan_op,
-    OutputT         initial_value)
+    InitialValueT   initial_value)
 {
+    // Use the initial value type for accumulation per P0571
+    using AccumT = InitialValueT;
+
     if (num_items > 0)
     {
-        OutputT val         = h_in[0];
-        h_reference[0]      = initial_value;
-        OutputT inclusive   = scan_op(initial_value, val);
+        AccumT val         = static_cast<AccumT>(h_in[0]);
+        h_reference[0]     = initial_value;
+        AccumT inclusive   = scan_op(initial_value, val);
 
         for (int i = 1; i < num_items; ++i)
         {
-            val = h_in[i];
-            h_reference[i] = inclusive;
+            val = static_cast<AccumT>(h_in[i]);
+            h_reference[i] = static_cast<OutputT>(inclusive);
             inclusive = scan_op(inclusive, val);
         }
     }
@@ -582,16 +586,20 @@ void Solve(
     ScanOpT         scan_op,
     NullType)
 {
+    // When no initial value type is supplied, use InputT for accumulation
+    // per P0571
+    using AccumT = typename std::iterator_traits<InputIteratorT>::value_type;
+
     if (num_items > 0)
     {
-        OutputT inclusive   = h_in[0];
-        h_reference[0]      = inclusive;
+        AccumT inclusive    = h_in[0];
+        h_reference[0]      = static_cast<OutputT>(inclusive);
 
         for (int i = 1; i < num_items; ++i)
         {
-            OutputT val = h_in[i];
+            AccumT val = h_in[i];
             inclusive = scan_op(inclusive, val);
-            h_reference[i] = inclusive;
+            h_reference[i] = static_cast<OutputT>(inclusive);
         }
     }
 }
@@ -746,7 +754,22 @@ void TestPointer(
 
     // Initialize problem and solution
     Initialize(gen_mode, h_in, num_items);
-    Solve(h_in, h_reference, num_items, scan_op, initial_value);
+
+    // If the output type is primitive and the operator is cub::Sum, the test
+    // dispatcher throws away scan_op and initial_value for exclusive scan.
+    // Without an initial_value arg, the accumulator switches to the input value
+    // type.
+    // Do the same thing here:
+    if (Traits<OutputT>::PRIMITIVE &&
+        Equals<ScanOpT, cub::Sum>::VALUE &&
+        !Equals<InitialValueT, NullType>::VALUE)
+    {
+      Solve(h_in, h_reference, num_items, cub::Sum{}, InputT{});
+    }
+    else
+    {
+      Solve(h_in, h_reference, num_items, scan_op, initial_value);
+    }
 
     // Allocate problem device arrays
     InputT *d_in = NULL;