<ranges>: views::counted::_Choose() misses difference casting for contiguous_iterator case

[hewillk]

STL/stl/inc/ranges

Lines 4214 to 4220 in 89ca073

	template <class _It>
	_NODISCARD static consteval _Choice_t<_St> _Choose() noexcept {
	using _Decayed = decay_t<_It>;
	_STL_INTERNAL_STATIC_ASSERT(input_or_output_iterator<_Decayed>);
	if constexpr (contiguous_iterator<_Decayed>) {
	return {_St::_Span,
	noexcept(span(_STD to_address(_STD declval<_It>()), iter_difference_t<_Decayed>{}))};

The second difference argument passed into span misses the size_t casting, which would cause a hard error if it is not implicitly-convertible to size_t.
testcase:

#include <ranges>

struct ContiguousIter {
  using iterator_category = std::contiguous_iterator_tag;
  using difference_type = std::_Signed128;
  using element_type = char;
  element_type& operator*() const;
  ContiguousIter& operator++();
  ContiguousIter operator++(int);
  ContiguousIter& operator--();
  ContiguousIter operator--(int);
  ContiguousIter& operator+=(difference_type);
  ContiguousIter& operator-=(difference_type);
  element_type* operator->() const;
  element_type& operator[](difference_type) const;
  friend ContiguousIter operator+(ContiguousIter, difference_type);
  friend ContiguousIter operator+(difference_type, ContiguousIter);
  friend ContiguousIter operator-(ContiguousIter, difference_type);
  friend difference_type operator-(ContiguousIter, ContiguousIter);
  auto operator<=>(const ContiguousIter&) const = default;
};

int main() {
  ContiguousIter it;
  std::ranges::contiguous_range auto r = std::views::counted(it, 42); // boom
}

https://godbolt.org/z/e1hv1o6h8

This can be a good first issue.

[frederick-vs-ja]

Looks like that we can just use span(_STD to_address(_STD declval<_It>()), size_t{}).

[CaseyCarter]

to_address is always noexcept. The conversion of an iterator difference type to size_t is always noexcept. Our span constructor that takes (pointer, size_t) is always noexcept. I think we can just use true.

[hewillk]

STL/stl/inc/ranges

Lines 4224 to 4227 in 89ca073

	} else if constexpr (constructible_from<_Decayed, _It>) {
	return {_St::_Subrange_counted,
	noexcept(subrange(
	counted_iterator(_STD declval<_It>(), iter_difference_t<_Decayed>{}), default_sentinel))};

In addition, counted_iterator(_STD declval<_It>(), ...) here may not be well-formed for some pathological cases such as I with an explicit copy constructor, i.e. explicit I(const I&).

[CaseyCarter]

std::copy_constructible and std::move_constructible have conversion requirements as well as construction requirements to disallow types whose pertinent constructors are explicit.

[hewillk]

std::copy_constructible and std::move_constructible have conversion requirements as well as construction requirements to disallow types whose pertinent constructors are explicit.

Sorry, what I meant here is that if _It has an explicit copy constructor and is an lvalue, then there will be a hard error in counted_iterator(_STD declval<_It>(), ...).

[CaseyCarter]

Sorry, what I meant here is that if _It has an explicit copy constructor and is an lvalue, then there will be a hard error in counted_iterator(_STD declval<_It>(), ...).

I see! Yes, we don't want the program to be ill-formed for the wrong reason (EDIT: Nope, it shouldn't be ill-formed at all in this case, Casey); we should be checking convertible_to<_It, _Decayed> in _Choose instead of constructible_from<_Decayed, _It>. Thanks for spelling it out for me.

[frederick-vs-ja]

I thing there're weird iterator types indicating that we should use is_convertible_v (Godbolt link).

#include <ranges>

struct I {
  using difference_type = int;
  I() = default;
  I(I&&) = default;
  I& operator=(I&&) = default;

  explicit I(const I&) = delete;
  template<int = 0>
  I(const I&);

  int operator*();
  I& operator++();
  void operator++(int);
};

int main() {
  I it;
  (void) std::views::counted(it, 5);   // currently rejected
  (void) std::counted_iterator(it, 5); // currently OK
}

Moreover, it's a bit unfortrunate that counted_iterator's corresponding constructor takes the iterator argument by value, which makes views::counted (at least sometimes) need an additional move. Should we add some internal mechanisms to achieve strict expression-equivalence?