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4021. mdspan::is_always_meow() should be noexcept

Section: 23.7.3.6.1 [mdspan.mdspan.overview] Status: New Submitter: Stephan T. Lavavej Opened: 2023-12-07 Last modified: 2025-10-21

Priority: 3

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Discussion:

It can easily be proven that mdspan::is_always_meow() is preconditionless and can never throw exceptions. microsoft/STL has already shipped a noexcept-strengthened implementation, libc++ would like to (see llvm/llvm-project#74254), and together we believe that the Standard should simply mandate noexcept so users can rely on this. The proof is:

• N4964 23.7.3.6.1 [mdspan.mdspan.overview]/3:

  LayoutPolicy shall meet the layout mapping policy requirements (23.7.3.4.3 [mdspan.layout.policy.reqmts]) […]

• 23.7.3.4.3 [mdspan.layout.policy.reqmts]/1:

  A type MP meets the layout mapping policy requirements if for a type E that is a specialization of extents, MP::mapping<E> is valid and denotes a type X that meets the layout mapping requirements (23.7.3.4.2 [mdspan.layout.reqmts]) […]

• 23.7.3.4.2 [mdspan.layout.reqmts]/1, /1.5, /22, /24, /26:

  A type M meets the layout mapping requirements if

  […]

  1. — the following types and expressions are well-formed and have the specified semantics.

     […]

     M::is_always_unique()

     Result: A constant expression (7.7 [expr.const]) of type bool.

     […]

     M::is_always_exhaustive()

     Result: A constant expression (7.7 [expr.const]) of type bool.

     […]

     M::is_always_strided()

     Result: A constant expression (7.7 [expr.const]) of type bool.

Constant expressions can surely be obtained without throwing exceptions.

[2025-10-21; Reflector poll.]

Set priority to 3 after reflector poll.

Use if consteval may lead to exception thrown from invocation of is_always_meow, when evaluated at runtime.

Design intent was for is_always_meow functions to always produce value at compile time. Layout requirements may need to be updated.

Separately, 16.3.2.4 [structure.specifications] says that Result: is just the type and value category; not "constant expression" - this matters because we need to require a prvalue constant expression here.

Proposed resolution:

This wording is relative to N4964.

1. Modify 23.7.3.6.1 [mdspan.mdspan.overview], class template mdspan synopsis, as indicated:

   […]
   static constexpr bool is_always_unique() noexcept
     { return mapping_type::is_always_unique(); }
   static constexpr bool is_always_exhaustive() noexcept
     { return mapping_type::is_always_exhaustive(); }
   static constexpr bool is_always_strided() noexcept
     { return mapping_type::is_always_strided(); }
   […]