From 4fd3a9ac56bc751916406998c08e96c9f71d637b Mon Sep 17 00:00:00 2001
From: Christian Trott <crtrott@sandia.gov>
Date: Fri, 31 May 2024 15:44:51 -0600
Subject: [PATCH] Avoid use of zero-length carray

---
 .../__p0009_bits/layout_stride.hpp            | 20 +++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/include/experimental/__p0009_bits/layout_stride.hpp b/include/experimental/__p0009_bits/layout_stride.hpp
index 53e47739..ded3156e 100644
--- a/include/experimental/__p0009_bits/layout_stride.hpp
+++ b/include/experimental/__p0009_bits/layout_stride.hpp
@@ -202,9 +202,17 @@ struct layout_stride {
         return __strides_storage_t{static_cast<index_type>(s[Idxs])...};
       }
 
-      template<class IntegralType>
+      MDSPAN_TEMPLATE_REQUIRES(
+        class IntegralType,
+        // The is_convertible condition is added to make sfinae valid
+        // the extents_type::rank() > 0 is added to avoid use of non-standard zero length c-array
+        (std::is_convertible<IntegralType, typename extents_type::index_type>::value && (extents_type::rank() > 0))
+      )
       MDSPAN_INLINE_FUNCTION
-      static constexpr const __strides_storage_t fill_strides(mdspan_non_standard_tag, const IntegralType (&s)[extents_type::rank()]) {
+      // despite the requirement some compilers still complain about zero length array during parsing
+      // making it length 1 now, but since the thing can't be instantiated due to requirement the actual
+      // instantiation of strides_storage will not fail despite mismatching length
+      static constexpr const __strides_storage_t fill_strides(mdspan_non_standard_tag, const IntegralType (&s)[extents_type::rank()>0?extents_type::rank():1]) {
         return __strides_storage_t{static_cast<index_type>(s[Idxs])...};
       }
 
@@ -325,7 +333,8 @@ struct layout_stride {
         // MSVC 19.32 does not like using index_type here, requires the typename Extents::index_type
         // error C2641: cannot deduce template arguments for 'MDSPAN_IMPL_STANDARD_NAMESPACE::layout_stride::mapping'
         _MDSPAN_TRAIT(std::is_convertible, const std::remove_const_t<IntegralTypes>&, typename Extents::index_type) &&
-        _MDSPAN_TRAIT(std::is_nothrow_constructible, typename Extents::index_type, const std::remove_const_t<IntegralTypes>&)
+        _MDSPAN_TRAIT(std::is_nothrow_constructible, typename Extents::index_type, const std::remove_const_t<IntegralTypes>&) &&
+        (Extents::rank() > 0)
       )
     )
     MDSPAN_INLINE_FUNCTION
@@ -333,7 +342,10 @@ struct layout_stride {
     mapping(
       mdspan_non_standard_tag,
       extents_type const& e,
-      IntegralTypes (&s)[extents_type::rank()]
+      // despite the requirement some compilers still complain about zero length array during parsing
+      // making it length 1 now, but since the thing can't be instantiated due to requirement the actual
+      // instantiation of strides_storage will not fail despite mismatching length
+      IntegralTypes (&s)[extents_type::rank()>0?extents_type::rank():1]
     ) noexcept
 #if defined(_MDSPAN_USE_ATTRIBUTE_NO_UNIQUE_ADDRESS)
       : __members{