Section: 24.7.3.6.1 [mdspan.mdspan.overview] Status: New Submitter: Stephan T. Lavavej Opened: 2023-12-07 Last modified: 2023-12-10 10:18:59 UTC
Priority: Not Prioritized
View all issues with New status.
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 24.7.3.6.1 [mdspan.mdspan.overview]/3:
LayoutPolicy shall meet the layout mapping policy requirements (24.7.3.4.3 [mdspan.layout.policy.reqmts]) […]
24.7.3.4.3 [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 (24.7.3.4.2 [mdspan.layout.reqmts]) […]
24.7.3.4.2 [mdspan.layout.reqmts]/1, /1.5, /22, /24, /26:
A type M meets the layout mapping requirements if
[…]
— 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.
Proposed resolution:
This wording is relative to N4964.
Modify 24.7.3.6.1 [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(); }
[…]