<format>: in_range apply on non-integrals triggers a static_assert

[hewillk]

Yap, another annoying difference-type issue.

STL/stl/inc/format

Lines 3283 to 3290 in 89ca073

	template <class _CharT, _RANGES input_range _Range, class _FormatContext>
	void _Range_formatter_format_as_string(_Range&& _Rng, _FormatContext& _Ctx, const bool _Debug) {
	if constexpr (_RANGES contiguous_range<_Range>) {
	const auto _Size = _STD _To_unsigned_like(_RANGES distance(_Rng));
	if (!_STD in_range<size_t>(_Size)) [[unlikely]] {
	_Throw_format_error("Formatted range is too long.");
	}

_Size may be of integer-class type such as _Signed128, which currently does not work with in_range as it Mandates standard or extended integer types:

#include <ranges>
#include <print>

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::ranges::subrange(it, it + 42);
  std::println("{:s}", r); // fire
}

Note that this is not reproducible on Godbolt as the STL version used by MSVC does not yet support formatting ranges. (Is there any way to reproduce this online?)

[frederick-vs-ja]

Actually, I don't think a contiguous_range can have size not representable in size_t. The contiguity of a range is defined via pointer arithmetic, which relies on array objects, so a contiguous range is empty, denotes contiguous array elements, or denotes a single non-array-element object.

Since the object model of C++ doesn't allow arrays of more than SIZE_MAX elements, I think this branch is essentially dead.

Edit: It seems that one might pass an invalid range of illegally huge size, which is UB. Perhaps we can catch this in debug modes.

[hewillk]

Actually, I don't think a contiguous_range can have size not representable in size_t. The contiguity of a range is defined via pointer arithmetic, which relies on array objects, so a contiguous range is empty, denotes contiguous array elements, or denotes a single non-array-element object.

Agreed. It seems reasonable that contiguous_range requires that the difference_type must be ptrdiff_t or at least the same width as ptrdiff_t. Although current standards do not require this.