https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95474
--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> --- 4.0 had a clear "A variable that is part of another variable (such as a field of a structure) but is not an array element or an array section cannot appear in a map clause." restriction, this has been lifted in 4.5 to allow the var.xyz style mapping, but it still didn't allow general expressions as the bases of array sections. Only 5.0 allows arbitrary expressions there.
