Fix regression in transform_inclusive_scan.

We were deducing a reference type when we needed a value type for the
transform iterator instantition.

Fixes #1332 and adds a regression test.

testing/transform_scan.cu

@@ -347,3 +347,55 @@ struct TestTransformScanToDiscardIterator
 };
 VariableUnitTest<TestTransformScanToDiscardIterator, IntegralTypes> TestTransformScanToDiscardIteratorInstance;
 
+// Regression test for https://github.com/NVIDIA/thrust/issues/1332
+// The issue was the internal transform_input_iterator_t created by the
+// transform_inclusive_scan implementation was instantiated using a reference
+// type for the value_type.
+template <typename T>
+void TestValueCategoryDeduction()
+{
+    thrust::device_vector<T> vec;
+
+    T a_h[10] = {5, 0, 5, 8, 6, 7, 5, 3, 0, 9};
+    vec.assign((T*)a_h, a_h + 10);
+
+
+    thrust::transform_inclusive_scan(thrust::device,
+                                     vec.cbegin(),
+                                     vec.cend(),
+                                     vec.begin(),
+                                     thrust::identity<>{},
+                                     thrust::maximum<>{});
+
+    ASSERT_EQUAL(T{5}, vec[0]);
+    ASSERT_EQUAL(T{5}, vec[1]);
+    ASSERT_EQUAL(T{5}, vec[2]);
+    ASSERT_EQUAL(T{8}, vec[3]);
+    ASSERT_EQUAL(T{8}, vec[4]);
+    ASSERT_EQUAL(T{8}, vec[5]);
+    ASSERT_EQUAL(T{8}, vec[6]);
+    ASSERT_EQUAL(T{8}, vec[7]);
+    ASSERT_EQUAL(T{8}, vec[8]);
+    ASSERT_EQUAL(T{9}, vec[9]);
+
+    vec.assign((T*)a_h, a_h + 10);
+    thrust::transform_exclusive_scan(thrust::device,
+                                     vec.cbegin(),
+                                     vec.cend(),
+                                     vec.begin(),
+                                     thrust::identity<>{},
+                                     T{},
+                                     thrust::maximum<>{});
+
+    ASSERT_EQUAL(T{0}, vec[0]);
+    ASSERT_EQUAL(T{5}, vec[1]);
+    ASSERT_EQUAL(T{5}, vec[2]);
+    ASSERT_EQUAL(T{5}, vec[3]);
+    ASSERT_EQUAL(T{8}, vec[4]);
+    ASSERT_EQUAL(T{8}, vec[5]);
+    ASSERT_EQUAL(T{8}, vec[6]);
+    ASSERT_EQUAL(T{8}, vec[7]);
+    ASSERT_EQUAL(T{8}, vec[8]);
+    ASSERT_EQUAL(T{8}, vec[9]);
+}
+DECLARE_GENERIC_UNITTEST(TestValueCategoryDeduction);

thrust/system/cuda/detail/transform_scan.h

@@ -50,17 +50,19 @@ transform_inclusive_scan(execution_policy<Derived> &policy,
                          TransformOp                transform_op,
                          ScanOp                     scan_op)
 {
-  // Use the input iterator's value type per https://wg21.link/P0571
+  // Use the transformed input iterator's value type per https://wg21.link/P0571
   using input_type = typename thrust::iterator_value<InputIt>::type;
 #if THRUST_CPP_DIALECT < 2017
   using result_type = typename std::result_of<TransformOp(input_type)>::type;
 #else
   using result_type = std::invoke_result_t<TransformOp, input_type>;
 #endif
 
+  using value_type = typename std::remove_reference<result_type>::type;
+
   typedef typename iterator_traits<InputIt>::difference_type size_type;
   size_type num_items = static_cast<size_type>(thrust::distance(first, last));
-  typedef transform_input_iterator_t<result_type,
+  typedef transform_input_iterator_t<value_type,
                                      InputIt,
                                      TransformOp>
       transformed_iterator_t;
@@ -88,7 +90,7 @@ transform_exclusive_scan(execution_policy<Derived> &policy,
                          ScanOp                     scan_op)
 {
   // Use the initial value type per https://wg21.link/P0571
-  using result_type = InitialValueType;
+  using result_type = typename std::remove_reference<InitialValueType>::type;
 
   typedef typename iterator_traits<InputIt>::difference_type size_type;
   size_type num_items = static_cast<size_type>(thrust::distance(first, last));

thrust/system/detail/generic/transform_scan.inl

@@ -51,10 +51,11 @@ __host__ __device__
   // Use the input iterator's value type per https://wg21.link/P0571
   using InputType = typename thrust::iterator_value<InputIterator>::type;
 #if THRUST_CPP_DIALECT < 2017
-  using ValueType = typename std::result_of<UnaryFunction(InputType)>::type;
+  using ResultType = typename std::result_of<UnaryFunction(InputType)>::type;
 #else
-  using ValueType = std::invoke_result_t<UnaryFunction, InputType>;
+  using ResultType = std::invoke_result_t<UnaryFunction, InputType>;
 #endif
+  using ValueType = typename std::remove_reference<ResultType>::type;
 
   thrust::transform_iterator<UnaryFunction, InputIterator, ValueType> _first(first, unary_op);
   thrust::transform_iterator<UnaryFunction, InputIterator, ValueType> _last(last, unary_op);
@@ -79,7 +80,7 @@ __host__ __device__
                                           AssociativeOperator binary_op)
 {
   // Use the initial value type per https://wg21.link/P0571
-  using ValueType = InitialValueType;
+  using ValueType = typename std::remove_reference<InitialValueType>::type;
 
   thrust::transform_iterator<UnaryFunction, InputIterator, ValueType> _first(first, unary_op);
   thrust::transform_iterator<UnaryFunction, InputIterator, ValueType> _last(last, unary_op);