From Jason Turner

[mdspan.layout.leftpad.cons]

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+ ###### Constructors <a id="mdspan.layout.leftpad.cons">[[mdspan.layout.leftpad.cons]]</a>
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+
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+ ``` cpp
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+ constexpr mapping(const extents_type& ext);
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+ ```
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+
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+ *Preconditions:*
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+
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+ - The size of the multidimensional index space `ext` is representable as
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+ a value of type `index_type`.
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+ - If *rank\_* is greater than one and `padding_value` does not equal
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+ `dynamic_extent`, then
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+ *`LEAST-MULTIPLE-AT-LEAST`*`(padding_value, ext.extent(0))` is
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+ representable as a value of type *index_type*.
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+ - If *rank\_* is greater than one and `padding_value` does not equal
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+ `dynamic_extent`, then the product of
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+ *`LEAST-MULTIPLE-AT-LEAST`*`(padding_value, ext.extent(0))` and all
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+ values `ext.extent(`k`)` with k in the range of \[`1`, *`rank_`*) is
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+ representable as a value of type `index_type`.
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+
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+ *Effects:*
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+
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+ - Direct-non-list-initializes *extents\_* with `ext`; and
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+ - if *rank\_* is greater than one, direct-non-list-initializes
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+ *stride-1*
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+ - with `ext.extent(0)` if padding_value is `dynamic_extent`,
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+ - otherwise with
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+ *`LEAST-MULTIPLE-AT-LEAST`*`(padding_value, ext.extent(0))`.
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+
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+ ``` cpp
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+ template<class OtherIndexType>
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+ constexpr mapping(const extents_type& ext, OtherIndexType pad);
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+ ```
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+
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+ *Constraints:*
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+
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+ - `is_convertible_v<OtherIndexType, index_type>` is `true`.
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+ - `is_nothrow_constructible_v<index_type, OtherIndexType>` is `true`.
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+
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+ *Preconditions:*
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+
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+ - `pad` is representable as a value of type `index_type`.
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+ - `extents_type::`*`index-cast`*`(pad)` is greater than zero.
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+ - If *rank\_* is greater than one, then
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+ *`LEAST-MULTIPLE-AT-LEAST`*`(pad, ext.extent(0))` is representable as
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+ a value of type `index_type.`
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+ - If *rank\_* is greater than one, then the product of
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+ *`LEAST-MULTIPLE-AT-LEAST`*`(pad, ext.extent(0))` and all values
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+ `ext.extent(`k`)` with k in the range of \[`1`, *`rank_`*) is
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+ representable as a value of type `index_type`.
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+ - If `padding_value` is not equal to `dynamic_extent`, `padding_value`
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+ equals `extents_type::`*`index-cast`*`(pad)`.
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+
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+ *Effects:* Direct-non-list-initializes *extents\_* with `ext`, and if
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+ *rank\_* is greater than one, direct-non-list-initializes *stride-1*
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+ with *`LEAST-MULTIPLE-AT-LEAST`*`(pad, ext.extent(0))`.
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+
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+ ``` cpp
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+ template<class OtherExtents>
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+ constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
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+ mapping(const layout_left::mapping<OtherExtents>& other);
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+ ```
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+
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+ *Constraints:* `is_constructible_v<extents_type, OtherExtents>` is
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+ `true`.
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+
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+ *Mandates:* If `OtherExtents::rank()` is greater than `1`, then
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+
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+ ``` cpp
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+ (static-padding-stride == dynamic_extent) ||
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+ (OtherExtents::static_extent(0) == dynamic_extent) ||
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+ (static-padding-stride == OtherExtents::static_extent(0))
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+ ```
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+
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+ is `true`.
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+
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+ *Preconditions:*
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+
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+ - If `extents_type::rank() > 1` is `true` and `padding_value` ==
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+ `dynamic_extent` is `false`, then `other.stride(1)` equals
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+ ``` cpp
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+ LEAST-MULTIPLE-AT-LEAST(padding_value,
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+ extents_type::index-cast(other.extents().extent(0)))
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+ ```
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+
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+ and
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+ - `other.required_span_size()` is representable as a value of type
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+ `index_type`.
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+
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+ *Effects:* Equivalent to `mapping(other.extents())`.
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+
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+ ``` cpp
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+ template<class OtherExtents>
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+ constexpr explicit(rank_ > 0)
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+ mapping(const layout_stride::mapping<OtherExtents>& other);
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+ ```
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+
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+ *Constraints:* `is_constructible_v<extents_type, OtherExtents>` is
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+ `true`.
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+
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+ *Preconditions:*
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+
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+ - If *rank\_* is greater than `1` and `padding_value` does not equal
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+ `dynamic_extent`, then `other.stride(1)` equals
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+ ``` cpp
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+ LEAST-MULTIPLE-AT-LEAST(padding_value,
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+ extents_type::index-cast(other.extents().extent(0)))
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+ ```
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+ - If *rank\_* is greater than 0, then `other.stride(0)` equals 1.
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+ - If *rank\_* is greater than 2, then for all r in the range \[`2`,
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+ *`rank_`*), `other.stride(r)` equals
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+ ``` cpp
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+ (other.extents().fwd-prod-of-extents(r) / other.extents().extent(0)) * other.stride(1)
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+ ```
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+ - `other.required_span_size()` is representable as a value of type
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+ *index_type*.
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+
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+ *Effects:*
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+
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+ - Direct-non-list-initializes *extents\_* with `other.extents()` and
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+ - if *rank\_* is greater than one, direct-non-list-initializes
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+ *stride-1* with `other.stride(1)`.
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+
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+ ``` cpp
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+ template<class LayoutLeftPaddedMapping>
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+ constexpr explicit(see below)
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+ mapping(const LayoutLeftPaddedMapping& other);
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+ ```
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+
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+ *Constraints:*
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+
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+ - *`is-layout-left-padded-mapping-of`*`<LayoutLeftPaddedMapping>` is
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+ `true`.
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+ - `is_constructible_v<extents_type, typename LayoutLeftPaddedMapping::extents_type>`
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+ is `true`.
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+
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+ *Mandates:* If *rank\_* is greater than 1, then
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+
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+ ``` cpp
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+ padding_value == dynamic_extent ||
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+ LayoutLeftPaddedMapping::padding_value == dynamic_extent ||
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+ padding_value == LayoutLeftPaddedMapping::padding_value
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+ ```
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+
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+ is `true`.
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+
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+ *Preconditions:*
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+
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+ - If *rank\_* is greater than 1 and `padding_value` does not equal
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+ `dynamic_extent`, then `other.stride(1)` equals
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+ ``` cpp
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+ LEAST-MULTIPLE-AT-LEAST(padding_value,
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+ extents_type::index-cast(other.extent(0)))
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+ ```
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+ - `other.required_span_size()` is representable as a value of type
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+ `index_type`.
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+
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+ *Effects:*
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+
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+ - Direct-non-list-initializes *extents\_* with `other.extents()` and
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+ - if *rank\_* is greater than one, direct-non-list-initializes
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+ *stride-1* with `other.stride(1)`.
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+
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+ *Remarks:* The expression inside `explicit` is equivalent to:
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+
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+ ``` cpp
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+ rank_> 1 &&
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+ (padding_value != dynamic_extent ||
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+ LayoutLeftPaddedMapping::padding_value == dynamic_extent)
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+ ```
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+
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+ ``` cpp
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+ template<class LayoutRightPaddedMapping>
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+ constexpr explicit(see below)
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+ mapping(const LayoutRightPaddedMapping& other) noexcept;
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+ ```
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+
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+ *Constraints:*
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+
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+ - *`is-layout-right-padded-mapping-of`*`<LayoutRightPaddedMapping>` is
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+ `true` or *`is-mapping-of`*`<layout_right, LayoutRightPaddedMapping>`
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+ is `true`.
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+ - *rank\_* equals zero or one.
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+ - `is_constructible_v<extents_type, typename LayoutRightPaddedMapping::extents_- type>`
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+ is `true`.
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+
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+ *Preconditions:* `other.required_span_size()` is representable as a
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+ value of type `index_type`.
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+
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+ *Effects:* Direct-non-list-initializes *extents\_* with
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+ `other.extents()`.
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+
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+ *Remarks:* The expression inside `explicit` is equivalent to:
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+
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+ ``` cpp
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+ !is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type>
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+ ```
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+
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+ [*Note 10*: Neither the input mapping nor the mapping to be constructed
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+ uses the padding stride in the rank-0 or rank-1 case, so the padding
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+ stride does not affect either the constraints or the
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+ preconditions. — *end note*]
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+