IT WORKS

Fix extent computation for std::complex.
It builds and passes tests.

NEXT STEPS:

* Re-test non-P3663 version
* Make it ill-formed to call submdspan_mapping with non-canonical
  slice types
* Make P3663 version only use constant_wrapper and canonical
  slice types in submdspan_mapping_impl functions

Author: mhoemmen

include/experimental/__p2630_bits/submdspan_extents.hpp

@@ -896,7 +896,7 @@ submdspan_canonicalize_one_slice(const extents<IndexType, Extents...>& exts, Sli
   else if constexpr (detail::is_std_complex<Slice>) {
     return strided_slice{
       .offset = canonical_ice<IndexType>(s.real()),
-      .extent = canonical_ice<IndexType>(s.imag()),
+      .extent = canonical_ice<IndexType>(s.imag() - s.real()),
       .stride = std::cw<IndexType(1)>
     };
   }