The constexpr mapping(const extents_type& ext); constructor currently has the following precondition (paragraph 2.2).
* [2.2]{.pnum} If `extents_type::rank()` is greater than one
and `padding_value` does not equal `dynamic_extent`,
then $\mathit{least\_multiple\_at\_least}($ `padding_value` $,$ `ext.extent(0)` $)$
is representable as a value of type `index_type`.
Given this wording, the following is not a precondition violation but will still produce offsets exceeding MAX_INT.
constexpr int e1 = MAX_INT / 3;
constexpr int s1 = 2*e1;
layout_left_padded<s1>::mapping<dextents<int, 2>> map(dextents<int, 2>{e1, 2});
@crtrott suggests the following wording.
* [2.2]{.pnum} If `extents_type::rank()` is greater than one
and `padding_value` does not equal `dynamic_extent`,
then $\mathit{least\_multiple\_at\_least}($ `padding_value` $,$ `ext.extent(0)` $)$
times the size of the multidimensional index space `ext` divided by `ext.extent(0)`
is representable as a value of type `index_type`.
Other
The Mandate "padding_value is representable as a value of type index_type" (without further qualifications) is wrong, because dynamic_extent is generally not representable as index_type.
index_type @_stride-rm2_@ = @_static-padding-stride_@ is an unsafe conversion when static-padding-stride is dynamic_extent and index_type is not size_t. The default member initializer shouldn't take effect in that case, but the syntax itself might still be incorrect.
Fixes for changes from LWG 2024/01/31
[mdspan.layout.leftpadded.cons]
The
constexpr mapping(const extents_type& ext);constructor currently has the following precondition (paragraph 2.2).Given this wording, the following is not a precondition violation but will still produce offsets exceeding
MAX_INT.@crtrott suggests the following wording.
Other
The Mandate "
padding_valueis representable as a value of typeindex_type" (without further qualifications) is wrong, becausedynamic_extentis generally not representable asindex_type.index_type @_stride-rm2_@ = @_static-padding-stride_@is an unsafe conversion whenstatic-padding-strideisdynamic_extentandindex_typeis notsize_t. The default member initializer shouldn't take effect in that case, but the syntax itself might still be incorrect.