transform_iterator fails with callables whose arguments are passed by non-const l-value ref

[jrhemstad]

Minimal reproducer: https://godbolt.org/z/s4oPhqx3W

This fails because it cannot deduce the return type of the callable.

A workaround is to explicitly define the result_type in the callable: https://godbolt.org/z/MTM8b1zW9

However, I believe a better solution is to fix the result type deduction.

It currently fails because it is ultimately attempting to do:

using result_type = std::invoke_result_t<ref, int>;

This fails due to using int as the argument type instead of int&.

This stems from using thrust::iterator_value on the adapted iterator here: https://github.com/NVIDIA/thrust/blob/d50708f1be248f4a42e81fdce360cf368990582e/thrust/iterator/detail/transform_iterator.inl#L39-L42

Instead of getting the value_type of the iterator, I believe it should use iterator_reference:

    typedef typename thrust::detail::ia_dflt_help<
      Reference,
      thrust::detail::result_of_adaptable_function<UnaryFunc(typename raw_reference<typename thrust::iterator_reference<Iterator>::type>>::type)>
    >::type reference;

Note the use of raw_reference to avoid issues with device_vector::iterator::reference.

Example: https://godbolt.org/z/8eePb5Kvc