[Bug bootstrap/89864] [9 regression] gcc fails to build/bootstrap with XCode 10.2

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89864

--- Comment #62 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Iain Sandoe from comment #61)
> (In reply to Jonathan Wakely from comment #60)
> > PR 81797 was the relevant bug, which apparently is still present for darwin
> > when using the buggy new APFS filesystem
> 
> so, from discussion on IRC, and notes above, this seems to be a known issue
> (with AFPS only) and the current remedy is to continue the bootstrap.
> 
> Let's consider that we have a viable work-around and then please can we wait
> for a response from Apple and the clang devs before making a final fix?
> 
> (i.e. this is enough of a work-around to prevent folks from being blocked
> right now)

BTW if you look at
https://godbolt.org/z/7nwoLN
and flip between C and C++, you will see that clang++ is not generating the
same code for _Atomic int x, as clang (C).  In fact, it looks like the
storage-class might well be accepted but ignored.  If that's the case, the the
right fix is probably to substitute volatile unconditionally for C++.

(that is, the right guard would be Jakub's suggestion from comment #48)