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<mdspan>
Section: 24.7.3 [views.multidim] Status: New Submitter: Hewill Kang Opened: 2023-10-11 Last modified: 2023-10-30
Priority: 3
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Discussion:
Currently, std::layout_meow::mapping::operator()
has the following definition (24.7.3.4.5.3 [mdspan.layout.left.obs]):
template<class... Indices> constexpr index_type operator()(Indices... i) const noexcept;-2- Constraints:
(2.1) —
sizeof...(Indices) == extents_type::rank()
istrue
,(2.2) —
(is_convertible_v<Indices, index_type> && ...)
istrue
, and(2.3) —
(is_nothrow_constructible_v<index_type, Indices> && ...)
istrue
.Preconditions:
Effects: Letextents_type::index-cast(i)
is a multidimensional index inextents_
(24.7.3.1 [mdspan.overview]).P
be a parameter pack such thatis_same_v<index_sequence_for<Indices...>, index_sequence<P...>>is
true
. Equivalent to:return ((static_cast<index_type>(i) * stride(P)) + ... + 0);
Above, is_convertible_v<Indices, index_type>
implies that index_type
can be constructed through
rvalue-qualified conversion operators. However, we cast the lvalue i
in the return statement, which makes the
expression possibly ill-formed. The same goes for extents_type::index-cast(i)
.
However, if we use std::move
before casting, this will result in the rvalue-qualified conversion operator
being called in Preconditions via extents_type::index-cast(i)
before the mapping index is actually calculated,
so that the expression may no longer be valid. And such an issue already exists in mdspan::operator[]
.
In addition, the variadic version of mdspan::operator[]
constraints
is_convertible_v<OtherIndexTypes, index_type>
, but its array
/span
version constraints
is_convertible_v<const OtherIndexType&, index_type>
.
mdspan[arr]
may not necessarily guarantee mdspan[arr[i]...]
.
I think we should unanimously require that custom indexes can be converted to index_type
via const
lvalue references, which eliminates the worry of conversion expiration.
[2023-10-30; Reflector poll]
Set priority to 3 after reflector poll.
"P4 - doesn't affect 'normal' uses of custom index types.
Only affects expert users that interface with the mapping directly,
because mdspan
does the conversions."
Proposed resolution: