try some stuff to see if msvc likes it

Author: ericniebler

include/stdexec/__detail/__meta.hpp

@@ -123,7 +123,7 @@ namespace stdexec {
 
   // `decltype(_Np)` instead of `auto` to work around NVHPC/EDG bug.
   template <auto _Np>
-  inline constexpr decltype(_Np) __v<__mconstant<_Np>> = _Np;
+  inline constexpr __mtypeof<_Np> __v<__mconstant<_Np>> = _Np;
 
   template <std::size_t _Np>
   inline constexpr std::size_t __v<__u8 (*)[_Np]> = _Np - 1; // see definition of __msize_t