transform_inclusive_scan broken with different value types

[rongou]

With previous versions of Thrust, we used to be able to pass in iterators of different value types to thrust::transform_inclusive_scan for the transform the inclusive scan stages, with the UnaryFunction converting between the two. For example, https://github.com/rapidsai/cudf/blob/branch-0.16/cpp/src/copying/concatenate.cu#L74. But this seems to be broken with the version at head:

/home/rou/src/cudf/cpp/build/_deps/thrust-src/thrust/iterator/transform_iterator.h(316): error: no suitable constructor exists to convert from "size_t" to "cudf::column_device_view"
          detected during:
            instantiation of "thrust::detail::transform_iterator_base<AdaptableUnaryFunction, Iterator, Reference, Value>::type::reference thrust::transform_iterator<AdaptableUnaryFunction, Iterator, Reference, Value>::dereference() const [with AdaptableUnaryFunction=lambda [](const auto &)->auto, Iterator=thrust::detail::normal_iterator<const cudf::column_device_view *>, Reference=cudf::column_device_view, Value=thrust::use_default]" 
/home/rou/src/cudf/cpp/build/_deps/thrust-src/thrust/iterator/iterator_facade.h(128): here
            instantiation of "Facade::reference thrust::iterator_core_access::dereference(const Facade &) [with Facade=thrust::transform_iterator<lambda [](const auto &)->auto, thrust::detail::normal_iterator<const cudf::column_device_view *>, cudf::column_device_view, thrust::use_default>]" 
/home/rou/src/cudf/cpp/build/_deps/thrust-src/thrust/iterator/iterator_facade.h(310): here
            instantiation of "thrust::iterator_facade<Derived, Value, System, Traversal, Reference, Difference>::reference thrust::iterator_facade<Derived, Value, System, Traversal, Reference, Difference>::operator*() const [with Derived=thrust::transform_iterator<lambda [](const auto &)->auto, thrust::detail::normal_iterator<const cudf::column_device_view *>, cudf::column_device_view, thrust::use_default>, Value=cudf::column_device_view, System=thrust::host_system_tag, Traversal=std::random_access_iterator_tag, Reference=cudf::column_device_view, Difference=std::ptrdiff_t]" 
/home/rou/src/cudf/cpp/build/_deps/thrust-src/thrust/system/detail/sequential/scan.h(67): here
            instantiation of "OutputIterator thrust::system::detail::sequential::inclusive_scan(thrust::system::detail::sequential::execution_policy<DerivedPolicy> &, InputIterator, InputIterator, OutputIterator, BinaryFunction) [with DerivedPolicy=thrust::system::cpp::detail::par_t, InputIterator=thrust::transform_iterator<lambda [](const auto &)->auto, thrust::detail::normal_iterator<const cudf::column_device_view *>, cudf::column_device_view, thrust::use_default>, OutputIterator=thrust::detail::normal_iterator<size_t *>, BinaryFunction=thrust::plus<size_t>]" 
/home/rou/src/cudf/cpp/build/_deps/thrust-src/thrust/detail/scan.inl(63): here
            instantiation of "OutputIterator thrust::inclusive_scan(const thrust::detail::execution_policy_base<DerivedPolicy> &, InputIterator, InputIterator, OutputIterator, AssociativeOperator) [with DerivedPolicy=thrust::system::cpp::detail::par_t, InputIterator=thrust::transform_iterator<lambda [](const auto &)->auto, thrust::detail::normal_iterator<const cudf::column_device_view *>, cudf::column_device_view, thrust::use_default>, OutputIterator=thrust::detail::normal_iterator<size_t *>, AssociativeOperator=thrust::plus<size_t>]" 
/home/rou/src/cudf/cpp/build/_deps/thrust-src/thrust/system/detail/generic/transform_scan.inl(57): here
            instantiation of "OutputIterator thrust::system::detail::generic::transform_inclusive_scan(thrust::execution_policy<ExecutionPolicy> &, InputIterator, InputIterator, OutputIterator, UnaryFunction, BinaryFunction) [with ExecutionPolicy=thrust::system::cpp::detail::par_t, InputIterator=thrust::detail::normal_iterator<const cudf::column_device_view *>, OutputIterator=thrust::detail::normal_iterator<size_t *>, UnaryFunction=lambda [](const auto &)->auto, BinaryFunction=thrust::plus<size_t>]" 
/home/rou/src/cudf/cpp/build/_deps/thrust-src/thrust/detail/transform_scan.inl(47): here
            instantiation of "OutputIterator thrust::transform_inclusive_scan(const thrust::detail::execution_policy_base<DerivedPolicy> &, InputIterator, InputIterator, OutputIterator, UnaryFunction, AssociativeOperator) [with DerivedPolicy=thrust::system::cpp::detail::par_t, InputIterator=thrust::detail::normal_iterator<const cudf::column_device_view *>, OutputIterator=thrust::detail::normal_iterator<size_t *>, UnaryFunction=lambda [](const auto &)->auto, AssociativeOperator=thrust::plus<size_t>]" 
/home/rou/src/cudf/cpp/src/copying/concatenate.cu(74): here

Probably shouldn't assume the InputIterator and OutputIterator would have the same value type.

[dkolsen-pgi]

This is a bug in the C++ standard (which I think is now an LWG issue), which is repeated in P0571. For the transform_inclusive_scan overload that has no initial value, the intermediate type should be the result type of the transformation operation, not the value type of the input iterator.

[rongou]

Can we fix it in Thrust or do we have to stick with the standard?

[dkolsen-pgi]

It should definitely be fixed in Thrust. It is an obvious bug with an obvious fix.