P0009: LWG small group review 2022/06/21

@crtrott @dalg24 @nliber

• Adding empty() (see PR #262) will require an LEWG paper or NB comment

• noexcept additions (see PR #262) are OK (?)

• extents(const array&) and extents(span) constructors: Rename SizeType (local) template parameter to OtherSizeType, to prevent shadowing class template parameter

• Change "integral type other than bool" to "a signed or unsigned integer type" (to exclude character types)

• "is a representable value of type SizeType" -> "is representable as a value of type SizeType (2.2, above [mdspan.extents.helpers] -- also update every instance, e.g., [mdspan.extents.ctor] 2.2) (justification: it's not of type SizeType; it's of some other type)

• para 9, below extents(const array&): "Preconditions: ..." line is duplicated.

• para 9.2, below extents(const array&): Could remove the N is zero subcase, as it's redundant.

• para 11: extents(Integrals...) deduction guide: change size_type to size_t (justification: the class doesn't exist yet, so size_type doesn't exist)

• [mdspan.layout.reqmts]: for layout mapping operator(), we also need to say that this is less than or equal to size_t max, because size_type could be (e.g.,) __int128 and bigger than size_t.

• (Note on [mdspan.layout.reqmts]: Use of "Result" vs. "Returns" not consistent, but not bad to leave it.)

• [mdspan.layoutleft.ctor]: para 3 (actually should be 4): change "Preconditions" to "Precondition" (as there is only one precondition); also fix paragraph numbering (three paragraph 3s in this section)

• [mdspan.layoutleft.obs]: Para 3 (Preconditions on operator()(Indices... i)): It's just one Precondition, and extents_ should be code font, not italics.

• [mdspan.layoutright]: generally apply above mdspan.layoutleft.* changes to corresponding paragraphs

• [mdspan.layoutstride]: In synopsis and mapping(const extents_type&, span<SizeType, rank_>), etc.: use OtherSizeType instead of SizeType for consistency

• [mdspan.layoutstride]: Change constexpr span<const size_type, rank_> strides() to return array (by value) instead of span, as it was in R16.

• [mdspan.layoutstride.ctor]: Change SizeType to OtherSizeType (see above synopsis notes).

• [mdspan.layoutstride.obs]: operator(): This wording forbids implementations from detecting out-of-bounds access for integer-like types, because it just static_casts to size_type. It needs to static_cast in order to support custom (class type) index types. Apply to this and to all layouts: num-cast<size_type>(i) is i if remove_cvref_t<decltype(i)> is an integer-like type, otherwise static_cast<size_type>(i).

• mdspan(CArray&) deduction guide: add spaces around == to avoid >== being considered as a single token.

• mdspan(Pointer&) deduction guide: Look again at whether we need lvalue reference; why do we forbid rvalue reference? Do temporary pointers work? Add a test that takes std::move(CArray) or std::move(Pointer).

• [mdspan.mdspan.members] para 2 and para 6 (operator[]): The same static_cast<size_type>(std::move(indices)) as above; use above num-cast wording. Move "Let P be a parameter pack" up sooner, so you can apply that wording.

• mdspan::size() needs to return size_t, not size_type (fixed in overview already, not in description).

• P2604R0: LWG probably won't like this non-diff form.

• P2604R0: "two uses of contiguous in prose text in notes will be replaced"

• P2599R1: For additions (not renaming), need to see proper diffs against the paper reviewed in this meeting (R17).

Begin by comparing the R17 paper with PR #262, then review the listed [mdspan] sections and the P2604R0 and P2599R1 notes. Done means resolving the open design questions, applying the requested wording and consistency changes, and producing proper diffs against the reviewed paper.