Proposed resolution:

This wording is relative to N4958.

1. Modify [mdspan.syn] as indicated:

   […]
   // [mdspan.submdspan], submdspan creation
   template<class OffsetType, class LengthType, class StrideType>
     struct strided_slice;
   
   template<class LayoutMapping>
     struct submdspan_mapping_result;
     
   struct full_extent_t { explicit full_extent_t() = default; };
   inline constexpr full_extent_t full_extent{};
   […]
   

submdspan uses a type called full_extent_t, but there isn't a definition for that type.

It appears that full_extent_t (along with full_extent) was proposed in P0009 before submdspan was moved into its own paper, and its definition failed to be included in P2630 Submdspan.