Update scan accum / binary_op edgecase handling.

TBB's scan was implemented differently than the other backends, leading
to some failing unit tests.

This patch fixes these inconsistencies by making the following changes:

- Follow P0571's guidance regarding accumulator variable type.
  - https://wg21.link/P0571
  - The accumulator's type is now:
    - The type of the user-supplied initial value (if provided), or
    - The input iterator's value type if no initial value.
- Follow C++ standard guidance for default binary operator type.
  - https://eel.is/c++draft/exclusive.scan#1
  - Thrust binary/unary functors now specialize a default void template
    parameter. Types are deduced and forwarded transparently.
  - Updated the scan's default binary operator to the new
    `thrust::plus<>` specialization.
- The `intermediate_type_from_function_and_iterators` helper is no
  longer needed and has been removed.

Closes #1170.

testing/scan.cu

@@ -250,48 +250,49 @@ void TestScanMixedTypes(void)
 
     IntVector   int_output(4);
     FloatVector float_output(4);
-     
-    // float -> int should use using plus<int> operator by default
+
+    // float -> int should use plus<void> operator and float accumulator by default
     thrust::inclusive_scan(float_input.begin(), float_input.end(), int_output.begin());
-    ASSERT_EQUAL(int_output[0],  1);
-    ASSERT_EQUAL(int_output[1],  3);
-    ASSERT_EQUAL(int_output[2],  6);
-    ASSERT_EQUAL(int_output[3], 10);
-    
-    // float -> float with plus<int> operator (int accumulator)
+    ASSERT_EQUAL(int_output[0],  1); // in: 1.5 accum: 1.5f out: 1
+    ASSERT_EQUAL(int_output[1],  4); // in: 2.5 accum: 4.0f out: 4
+    ASSERT_EQUAL(int_output[2],  7); // in: 3.5 accum: 7.5f out: 7
+    ASSERT_EQUAL(int_output[3], 12); // in: 4.5 accum: 12.f out: 12
+
+    // float -> float with plus<int> operator (float accumulator)
     thrust::inclusive_scan(float_input.begin(), float_input.end(), float_output.begin(), thrust::plus<int>());
-    ASSERT_EQUAL(float_output[0],  1.5);
-    ASSERT_EQUAL(float_output[1],  3.0);
-    ASSERT_EQUAL(float_output[2],  6.0);
-    ASSERT_EQUAL(float_output[3], 10.0);
-    
-    // float -> int should use using plus<int> operator by default
+    ASSERT_EQUAL(float_output[0],  1.5f); // in: 1.5 accum: 1.5f out: 1.5f
+    ASSERT_EQUAL(float_output[1],  3.0f); // in: 2.5 accum: 3.0f out: 3.0f
+    ASSERT_EQUAL(float_output[2],  6.0f); // in: 3.5 accum: 6.0f out: 6.0f
+    ASSERT_EQUAL(float_output[3], 10.0f); // in: 4.5 accum: 10.f out: 10.f
+
+    // float -> int should use plus<void> operator and float accumulator by default
     thrust::exclusive_scan(float_input.begin(), float_input.end(), int_output.begin());
-    ASSERT_EQUAL(int_output[0], 0);
-    ASSERT_EQUAL(int_output[1], 1);
-    ASSERT_EQUAL(int_output[2], 3);
-    ASSERT_EQUAL(int_output[3], 6);
-    
-    // float -> int should use using plus<int> operator by default
+    ASSERT_EQUAL(int_output[0], 0); // out: 0.0f  in: 1.5 accum: 1.5f
+    ASSERT_EQUAL(int_output[1], 1); // out: 1.5f  in: 2.5 accum: 4.0f
+    ASSERT_EQUAL(int_output[2], 4); // out: 4.0f  in: 3.5 accum: 7.5f
+    ASSERT_EQUAL(int_output[3], 7); // out: 7.5f  in: 4.5 accum: 12.f
+
+    // float -> int should use plus<> operator and float accumulator by default
     thrust::exclusive_scan(float_input.begin(), float_input.end(), int_output.begin(), (float) 5.5);
-    ASSERT_EQUAL(int_output[0],  5);
-    ASSERT_EQUAL(int_output[1],  7);
-    ASSERT_EQUAL(int_output[2],  9);
-    ASSERT_EQUAL(int_output[3], 13);
-    
-    // int -> float should use using plus<float> operator by default
+    ASSERT_EQUAL(int_output[0],  5); // out: 5.5f  in: 1.5 accum: 7.0f
+    ASSERT_EQUAL(int_output[1],  7); // out: 7.0f  in: 2.5 accum: 9.5f
+    ASSERT_EQUAL(int_output[2],  9); // out: 9.5f  in: 3.5 accum: 13.0f
+    ASSERT_EQUAL(int_output[3], 13); // out: 13.f  in: 4.5 accum: 17.4f
+
+    // int -> float should use using plus<> operator and int accumulator by default
     thrust::inclusive_scan(int_input.begin(), int_input.end(), float_output.begin());
-    ASSERT_EQUAL(float_output[0],  1.0);
-    ASSERT_EQUAL(float_output[1],  3.0);
-    ASSERT_EQUAL(float_output[2],  6.0);
-    ASSERT_EQUAL(float_output[3], 10.0);
-
-    // int -> float should use using plus<float> operator by default
+    ASSERT_EQUAL(float_output[0],  1.f); // in: 1 accum: 1  out: 1
+    ASSERT_EQUAL(float_output[1],  3.f); // in: 2 accum: 3  out: 3
+    ASSERT_EQUAL(float_output[2],  6.f); // in: 3 accum: 6  out: 6
+    ASSERT_EQUAL(float_output[3], 10.f); // in: 4 accum: 10 out: 10
+
+    // int -> float + float init_value should use using plus<> operator and
+    // float accumulator by default
     thrust::exclusive_scan(int_input.begin(), int_input.end(), float_output.begin(), (float) 5.5);
-    ASSERT_EQUAL(float_output[0],  5.5);
-    ASSERT_EQUAL(float_output[1],  6.5);
-    ASSERT_EQUAL(float_output[2],  8.5);
-    ASSERT_EQUAL(float_output[3], 11.5);
+    ASSERT_EQUAL(float_output[0],  5.5f); // out: 5.5f  in: 1 accum: 6.5f
+    ASSERT_EQUAL(float_output[1],  6.5f); // out: 6.0f  in: 2 accum: 8.5f
+    ASSERT_EQUAL(float_output[2],  8.5f); // out: 8.0f  in: 3 accum: 11.5f
+    ASSERT_EQUAL(float_output[3], 11.5f); // out: 11.f  in: 4 accum: 15.5f
 }
 void TestScanMixedTypesHost(void)
 {