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- […]

[ 2020-07-17; Priority set to 3 in telecon ]

The requirements for the overloads of std::transform_inclusive_scan without an initial value incorrectly assume that the internal accumulator uses the iterator's value type, as it does for std::inclusive_scan, rather than the transformed type of the iterator's value type, as it was intended.

According to the standard, the following program is ill-formed as it requires std::string to be convertible to int:

auto vs = {0, 1, 2};
std::transform_inclusive_scan(
   vs.begin(), vs.end(),
   std::ostream_iterator<std::string>(std::cout, ";"),
   [](std::string x, std::string y) { return x + y; },
   [](int x) { return std::to_string(x); });

libstdc++ and Microsoft's STL accept the snippet, producing 0;01;012; as expected, libc++ strictly conforms to the standard and rejects it.

These constrains were introduced by P0574R1.