Issue 4272: For `rank == 0`, `layout_stride` is atypically convertible

Title: For `rank == 0`, `layout_stride` is atypically convertible
Status: wp
Section: [mdspan.layout]
Submitter: Luc Grosheintz

Created on 2025-06-02.00:00:00 last changed 1 month ago

Messages

Date: 2025-11-11.10:48:55

Proposed resolution:

This wording is relative to N5008.

[Drafting note: As drive-by fixes the edits for layout_left_padded<>::mapping and layout_right_padded<>::mapping also correct an editorial asymmetry between class header synopsis declaration form and prototype specification form of the corresponding constructors and adjust to the correct formatting of the exposition-only data member rank_.]

Modify [mdspan.layout.left.overview] as indicated:

namespace std {
  template<class Extents>
  class layout_left::mapping {
    […]
    // [mdspan.layout.left.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    
    constexpr mapping& operator=(const mapping&) noexcept = default;
    […]
  };
}

Modify [mdspan.layout.left.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(extents_type::rank() > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-13- Constraints: […]
-14- Preconditions: […]
-15- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

Modify [mdspan.layout.right.overview] as indicated:

namespace std {
  template<class Extents>
  class layout_right::mapping {
    […]
    // [mdspan.layout.right.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    
    constexpr mapping& operator=(const mapping&) noexcept = default;
    […]
  };
}

Modify [mdspan.layout.right.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(extents_type::rank() > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-13- Constraints: […]
-14- Preconditions: […]
-15- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

Modify [mdspan.layout.leftpad.overview] as indicated:

namespace std {
  template<size_t PaddingValue>
  template<class Extents>
  class layout_left_padded<PaddingValue>::mapping {
    […]
    // [mdspan.layout.leftpad.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    […]
  };
}

Modify [mdspan.layout.leftpad.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(rank_ > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);

-10- Constraints: […]
-11- Preconditions: […]
-12- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

template<class LayoutLeftPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutLeftPaddedMapping& other);

-13- Constraints: […]
[…]
-16- Remarks: The expression inside `explicit` is equivalent to:
!is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type> ||
rank_> 1 &&
(padding_value != dynamic_extent ||
 LayoutLeftPaddedMapping::padding_value == dynamic_extent)

Modify [mdspan.layout.rightpad.overview] as indicated:

namespace std {
  template<size_t PaddingValue>
  template<class Extents>
  class layout_right_padded<PaddingValue>::mapping {
    […]
    // [mdspan.layout.rightpad.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(rank_ > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    […]
  };
}

Modify [mdspan.layout.rightpad.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(rank_ > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);

-10- Constraints: […]
-11- Preconditions: […]
-12- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

template<class LayoutRightPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutRightPaddedMapping& other);

-13- Constraints: […]
[…]
-17- Remarks: The expression inside `explicit` is equivalent to:
!is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type> ||
rank_ > 1 &&
(padding_value != dynamic_extent ||
LayoutRightPaddedMapping::padding_value == dynamic_extent)

msg15533 (view)

Date: 2025-11-11.10:48:55

[ Kona 2025-11-08; Status changed: Immediate → WP. ]

msg14975 (view)

Date: 2025-11-05.21:30:43

[ Kona 2025-11-05; approved by LWG. Status changed: New → Immediate. ]

msg14867 (view)

Date: 2025-09-15.00:00:00

[ 2025-09-27, Tomasz Kamiński fixes constraints in constructors from padded layouts ]

msg14798 (view)

Date: 2025-06-15.00:00:00

[ 2025-06-20, Luc Grosheintz provides further wording improvements ]

This wording is relative to N5008.

[Drafting note: As drive-by fixes the edits for layout_left_padded<>::mapping and layout_right_padded<>::mapping also correct an editorial asymmetry between class header synopsis declaration form and prototype specification form of the corresponding constructors and adjust to the correct formatting of the exposition-only data member rank_.]

Modify [mdspan.layout.left.overview] as indicated:

namespace std {
  template<class Extents>
  class layout_left::mapping {
    […]
    // [mdspan.layout.left.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    
    constexpr mapping& operator=(const mapping&) noexcept = default;
    […]
  };
}

Modify [mdspan.layout.left.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(extents_type::rank() > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-13- Constraints: […]
-14- Preconditions: […]
-15- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

Modify [mdspan.layout.right.overview] as indicated:

namespace std {
  template<class Extents>
  class layout_right::mapping {
    […]
    // [mdspan.layout.right.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    
    constexpr mapping& operator=(const mapping&) noexcept = default;
    […]
  };
}

Modify [mdspan.layout.right.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(extents_type::rank() > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-13- Constraints: […]
-14- Preconditions: […]
-15- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

Modify [mdspan.layout.leftpad.overview] as indicated:

namespace std {
  template<size_t PaddingValue>
  template<class Extents>
  class layout_left_padded<PaddingValue>::mapping {
    […]
    // [mdspan.layout.leftpad.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    […]
  };
}

Modify [mdspan.layout.leftpad.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(rank_ > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);

-10- Constraints: […]
-11- Preconditions: […]
-12- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

template<class LayoutLeftPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutLeftPaddedMapping& other);

-13- Constraints: […]
[…]
-16- Remarks: The expression inside `explicit` is equivalent to:
!is_convertible_v<typename LayoutLeftPaddedMapping::extents_type, extents_type> &&
rank_> 1 &&
(padding_value != dynamic_extent ||
 LayoutLeftPaddedMapping::padding_value == dynamic_extent)

Modify [mdspan.layout.rightpad.overview] as indicated:

namespace std {
  template<size_t PaddingValue>
  template<class Extents>
  class layout_right_padded<PaddingValue>::mapping {
    […]
    // [mdspan.layout.rightpad.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(rank_ > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    […]
  };
}

Modify [mdspan.layout.rightpad.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(rank_ > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);

-10- Constraints: […]
-11- Preconditions: […]
-12- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

template<class LayoutRightPaddedMapping>
  constexpr explicit(see below)
    mapping(const LayoutRightPaddedMapping& other);

-13- Constraints: […]
[…]
-17- Remarks: The expression inside `explicit` is equivalent to:
!is_convertible_v<typename LayoutRightPaddedMapping::extents_type, extents_type> &&
rank_ > 1 &&
(padding_value != dynamic_extent ||
LayoutRightPaddedMapping::padding_value == dynamic_extent)

msg14779 (view)

Date: 2025-06-15.00:00:00

[ 2025-06-12; Reflector poll ]

Set priority to 2 after reflector poll.

This wording is relative to N5008.

[Drafting note: As drive-by fixes the edits for layout_left_padded<>::mapping and layout_right_padded<>::mapping also correct an editorial asymmetry between class header synopsis declaration form and prototype specification form of the corresponding constructors and adjust to the correct formatting of the exposition-only data member rank_.]

Modify [mdspan.layout.left.overview] as indicated:

namespace std {
  template<class Extents>
  class layout_left::mapping {
    […]
    // [mdspan.layout.left.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    
    constexpr mapping& operator=(const mapping&) noexcept = default;
    […]
  };
}

Modify [mdspan.layout.left.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(extents_type::rank() > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-13- Constraints: […]
-14- Preconditions: […]
-15- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

Modify [mdspan.layout.right.overview] as indicated:

namespace std {
  template<class Extents>
  class layout_right::mapping {
    […]
    // [mdspan.layout.right.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    
    constexpr mapping& operator=(const mapping&) noexcept = default;
    […]
  };
}

Modify [mdspan.layout.right.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(extents_type::rank() > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-13- Constraints: […]
-14- Preconditions: […]
-15- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(extents_type::rank() == 0 && is_convertible_v<OtherExtents, extents_type>)

Modify [mdspan.layout.leftpad.overview] as indicated:

namespace std {
  template<size_t PaddingValue>
  template<class Extents>
  class layout_left_padded<PaddingValue>::mapping {
    […]
    // [mdspan.layout.leftpad.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(extents_type::rank() > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    […]
  };
}

Modify [mdspan.layout.leftpad.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(rank_ > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-10- Constraints: […]
-11- Preconditions: […]
-12- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

Modify [mdspan.layout.rightpad.overview] as indicated:

namespace std {
  template<size_t PaddingValue>
  template<class Extents>
  class layout_right_padded<PaddingValue>::mapping {
    […]
    // [mdspan.layout.rightpad.cons], constructors
    […]
    template<class OtherExtents>
      constexpr explicit(rank_ > 0see below)
        mapping(const layout_stride::mapping<OtherExtents>&);
    […]
  };
}

Modify [mdspan.layout.rightpad.cons] as indicated:

template<class OtherExtents>
  constexpr explicit(rank_ > 0see below)
    mapping(const layout_stride::mapping<OtherExtents>& other);
-10- Constraints: […]
-11- Preconditions: […]
-12- Effects: […]
-?- Remarks: The expression inside `explicit` is equivalent to:
!(rank_ == 0 && is_convertible_v<OtherExtents, extents_type>)

msg14778 (view)

Date: 2025-06-14.09:46:28

Commonly, two layouts are considered convertible, if the underlying `extent_types` are convertible.

However, for the two ctors `layout_left::mapping(layout_stride::mapping)` and `layout_right::mapping(layout_stride::mapping)`, the condition is rank > 0. Therefore,

using E1 = std::extents<int>;
using E2 = std::extents<unsigned int>;

static_assert(std::is_convertible_v<
    std::layout_stride::mapping<E2>,
    std::layout_right::mapping<E1>
  >);

even though:

static_assert(!std::is_convertible_v<E2, E1>);

Moreover, for rank 0 `layout_stride` can be converted to any specialization of `layout_left` or `layout_right`; but not to every specialization of `layout_stride`.

History
Date	User	Action	Args
2025-11-11 10:48:55	admin	set	messages: + msg15668
2025-11-11 10:48:55	admin	set	status: immediate -> wp
2025-11-05 21:30:43	admin	set	messages: + msg15533
2025-11-05 21:30:43	admin	set	status: new -> immediate
2025-08-29 17:48:54	admin	set	messages: + msg14975
2025-06-28 12:53:59	admin	set	messages: + msg14867
2025-06-12 20:53:08	admin	set	messages: + msg14798
2025-06-06 17:05:10	admin	set	messages: + msg14779
2025-06-02 00:00:00	admin	create