Fix compile error in __serial_merge

[SergeyKopienko]

In this PR we fix compile errors in __serial_merge function for case when the types of __rng2 and __rng1 aren't the same:

/include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_merge.h:162:17: error: conditional expression is ambiguous; '__return_t' (aka 'const oneapi::dpl::__internal::tuple<unsigned long, unsigned long>') can be converted to 'decltype(this->make_reference(::std::declval<_tuple_ranges_t>(), __i, ::std::make_index_sequence<__num_ranges>()))' (aka 'tuple<const unsigned long &, const unsigned long &>') and vice versa
  162 |                 ? __rng2[__rng2_idx++]
      |                 ^ ~~~~~~~~~~~~~~~~~~~~
  163 |                 : __rng1[__rng1_idx++];
      |                   ~~~~~~~~~~~~~~~~~~~~
/include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_merge.h:198:21: note: in instantiation of function template specialization 'oneapi::dpl::__par_backend_hetero::__serial_merge<oneapi::dpl::__ranges::zip_view<oneapi::dpl::__ranges::all_view<unsigned long, sycl::access::mode::read>, oneapi::dpl::__ranges::all_view<unsigned long, sycl::access::mode::read>>, oneapi::dpl::__ranges::all_view<oneapi::dpl::__internal::tuple<unsigned long, unsigned long>, sycl::access::mode::read>, const oneapi::dpl::__ranges::zip_view<oneapi::dpl::__ranges::all_view<unsigned long, sycl::access::mode::write>, oneapi::dpl::__ranges::all_view<unsigned long, sycl::access::mode::write>>, unsigned int, dpct::internal::compare_key_fun<>>' requested here
  198 |                     __serial_merge(__rng1, __rng2, __rng3, __start.first, __start.second, __i_elem, __chunk, __n1, __n2,
      |                     ^
/include/oneapi/dpl/pstl/hetero/algorithm_impl_hetero.h:1821:37: note: in instantiation of function template specialization 'oneapi::dpl::__internal::__pattern_merge<oneapi::dpl::__internal::__device_backend_tag, oneapi::dpl::execution::device_policy<oneapi::dpl::__internal::__set_union_copy_case_2<SetUnionByKey>>, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, oneapi::dpl::__internal::tuple<unsigned long, unsigned long>>, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, dpct::internal::compare_key_fun<>>' requested here
 1821 |     return oneapi::dpl::__internal::__pattern_merge(
      |                                     ^
/include/oneapi/dpl/pstl/glue_algorithm_impl.h:1007:37: note: in instantiation of function template specialization 'oneapi::dpl::__internal::__pattern_set_union<oneapi::dpl::__internal::__device_backend_tag, oneapi::dpl::execution::device_policy<SetUnionByKey> &, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, dpct::internal::compare_key_fun<>>' requested here
 1007 |     return oneapi::dpl::__internal::__pattern_set_union(__dispatch_tag, ::std::forward<_ExecutionPolicy>(__exec),                                                                |                                     ^
/include/dpct/dpl_extras/algorithm.h:840:23: note: in instantiation of function template specialization 'oneapi::dpl::set_union<oneapi::dpl::execution::device_policy<SetUnionByKey> &, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, oneapi::dpl::zip_iterator<oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>, oneapi::dpl::__internal::sycl_iterator<sycl::access::mode::read_write, unsigned long>>, dpct::internal::compare_key_fun<>>' requested here
  840 |   auto ret_val = std::set_union(                                                               

This compile error has been introduced in the PR #1970

[danhoeflinger]

Fix looks good with Green CI.

Does testing cover this case or should we add a test to prevent regression?

[SergeyKopienko]

Does testing cover this case or should we add a test to prevent regression?

It's not covered by our tests.
And it's not simple different types of source data for merge, these two types may be converted from one another and back versa:

• please take a look for details at https://www.emmtrix.com/wiki/Clang:Diag/err_conditional_ambiguous_ovl#:~:text=In%20C%2B%2B%2C%20when%20an,expression%2C%20this%20error%20is%20triggered

But may this mean that we has some problem in the implementation of oneapi::dpl::__internal::tuple ?

I mean, it implements more then one conversion operators :

    // conversion to ::std::tuple with the same template arguments
    operator ::std::tuple<T1, T...>() const
    {
        static constexpr ::std::size_t __tuple_size = sizeof...(T) + 1;
        return to_std_tuple(*this, ::std::make_index_sequence<__tuple_size>());
    }

    // conversion to ::std::tuple with the different template arguments
    template <typename U1, typename... U>
    operator ::std::tuple<U1, U...>() const
    {
        constexpr ::std::size_t __tuple_size = sizeof...(T) + 1;
        return to_std_tuple(static_cast<tuple<U1, U...>>(*this), ::std::make_index_sequence<__tuple_size>());
    }

And it's looks like the root case of this compile error.
Sure, this PR will fix them. But is it enought?

[danhoeflinger]

But may this mean that we has some problem in the implementation of oneapi::dpl::__internal::tuple ?

Ah, interesting, you may be correct...
We would have to be very careful with any changes here.

I need to investigate about what implicit conversion is allowed between std::tuple<T...> and a differently but per-element convertible typed std::tuple<U...>. I wonder if we should only have one of these implicit conversions. The second option maybe should cover the first.

[SergeyKopienko]

I created new issue for the future investigations: #2017

[danhoeflinger]

The fix LGTM.

[MikeDvorskiy]

LGTM

[danhoeflinger]

Still LGTM after removal of parens.