extents::index-cast
weirdnessSection: 23.7.3.3.2 [mdspan.extents.expo] Status: New Submitter: Casey Carter Opened: 2023-11-29 Last modified: 2023-12-02
Priority: Not Prioritized
View all issues with New status.
Discussion:
The exposition-only static member index-cast
of extents
is specified as
(23.7.3.3.2 [mdspan.extents.expo]/9):
template<class OtherIndexType> static constexpr auto index-cast(OtherIndexType&& i) noexcept;-9- Effects:
(9.1) — If
OtherIndexType
is an integral type other thanbool
, then equivalent toreturn i;
,(9.2) — otherwise, equivalent to
return static_cast<index_type>(i);
.[Note 1: This function will always return an integral type other than
bool
. Since this function's call sites are constrained on convertibility ofOtherIndexType
toindex_type
, integer-class types can use thestatic_cast
branch without loss of precision. — end note]
This function returns T
when passed an rvalue of cv-unqualified integral type T
,
but index_type
when passed a cv-qualified and/or lvalue argument of any integral type. It
would seem more consistent and easier to reason about if 9.1 was instead conditional on
remove_cvref_t<OtherIndexType>
.
Proposed resolution:
This wording is relative to N4964.
Modify 23.7.3.3.2 [mdspan.extents.expo] as indicated:
template<class OtherIndexType> static constexpr auto index-cast(OtherIndexType&& i) noexcept;-9- Effects:
(9.1) — If
remove_cvref_t<OtherIndexType>
is an integral type other thanbool
, then equivalent toreturn i;
,(9.2) — otherwise, equivalent to
return static_cast<index_type>(i);
.[Note 1: This function will always return an integral type other than
bool
. Since this function's call sites are constrained on convertibility ofOtherIndexType
toindex_type
, integer-class types can use thestatic_cast
branch without loss of precision. — end note]