[R-SIG-Mac] Replication runtime error on MacOS build machines

[Jim Hester]

I maintain the fs package, which is currently failing _only_ on CRANs
MacOS build machine.
<https://www.r-project.org/nosvn/R.check/r-release-osx-x86_64/fs-00check.html>

I have been trying to replicate this error and unfortunately have been
unable to do so, the package works as expected locally (MacOS 10.12.6)
and for numerous other people using El Capitan (as used on CRAN's
system) <https://twitter.com/jimhester_/status/964513974942420994>. I
also tested it on a El Capitan VM without reproducing the failure.

The main difference I am aware of on CRAN's systems is the newer
(4.0.0) clang/llvm toolchain than that shipped with XCode. I tested
compiling / running fs with this toolchain however and did not
reproduce the crash.

If anyone has an idea of what could be causing the crash on CRAN's
system I would be grateful. I would really like to fix this so we can
have MacOS binaries for fs.

Jim