[Bug c++/68508] [5/6 Regression] Internal compiler error with parentheses around return value in C++14 with ASan enabled

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68508

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 36822
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36822&action=edit
gcc6-pr68508.patch

Untested GCC 6 fix.

So, my first patch for this was the gcc5-* one, just let the code ignore the
instrumentation.  But the second question is why are we actually instrumenting
this, the cast in that case is from A * to A &, so it is the same class.
And here it seems that DERIVED_FROM_P returns true even for the same type used
as type and base, plus in one of the two cases where the downcast routine is
called the caller already adjusts the op through build_base_path and we already
don't have the original type.  My understanding has been that we want to
instrument only downcasts, and A * to A & is not a downcast IMHO.  Though,
perhaps for GCC 5.x backport the first patch is safer.