PR c++/28907 points out that casting from a derived class to a base
class fails in some situations. The problem turned out to be a
missing use of value_embedded_offset. One peculiarity here is that,
if you managed to construct a pointer-to-derived with an embedded
offset of 0, the cast would work -- for example, one of the two new
tests here passes without the patch.
This embedded offset stuff is an endless source of bugs. I wonder if
it's possible to get rid of it somehow.
Regression tested on x86-64 Fedora 34.
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28907
