Proposed resolution:

This wording is relative to N4964.

1. Modify [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(); }
   […]
   

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 [mdspan.mdspan.overview]/3:

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

• [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 ([mdspan.layout.reqmts]) […]

• [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 ([expr.const]) of type bool.

     […]

     M::is_always_exhaustive()

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

     […]

     M::is_always_strided()

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

Constant expressions can surely be obtained without throwing exceptions.