Proposed resolution:

This wording is relative to N4861.

[Drafting note: Current implementations that accept the code, do some form of auto acc = unary_op(*first);, therefore the following proposed wording uses decay_t instead of e.g. remove_cvref_t.]

1. Modify [transform.inclusive.scan] as indicated:

   template<class InputIterator, class OutputIterator,
            class BinaryOperation, class UnaryOperation>
     constexpr OutputIterator
       transform_inclusive_scan(InputIterator first, InputIterator last,
                                OutputIterator result,
                                BinaryOperation binary_op, UnaryOperation unary_op);
   template<class ExecutionPolicy,
            class ForwardIterator1, class ForwardIterator2,
            class BinaryOperation, class UnaryOperation>
     ForwardIterator2
       transform_inclusive_scan(ExecutionPolicy&& exec,
                                ForwardIterator1 first, ForwardIterator1 last,
                                ForwardIterator2 result,
                                BinaryOperation binary_op, UnaryOperation unary_op);
   template<class InputIterator, class OutputIterator,
            class BinaryOperation, class UnaryOperation, class T>
     constexpr OutputIterator
       transform_inclusive_scan(InputIterator first, InputIterator last,
                                OutputIterator result,
                                BinaryOperation binary_op, UnaryOperation unary_op,
                                T init);
   template<class ExecutionPolicy,
            class ForwardIterator1, class ForwardIterator2,
            class BinaryOperation, class UnaryOperation, class T>
     ForwardIterator2
       transform_inclusive_scan(ExecutionPolicy&& exec,
                                ForwardIterator1 first, ForwardIterator1 last,
                                ForwardIterator2 result,
                                BinaryOperation binary_op, UnaryOperation unary_op,
                                T init);
   

   -1- Let U be the value type of decltype(first)decay_t<decltype(unary_op(*first))>.

   -2- […]