Executing this
> exec {INDOC}<<----
> this is a test
> of the emergency
> broadcast system
> ---

reliably segfaults on
ZSH_VERSION=5.2, OpenBSD 6.0 GENERIC.MP#2319 amd64
ZSH_VERSION=5.3.1, Darwin 16.3.0; root:xnu-3789.31.2~1/RELEASE_X86_64 x86_64
ZSH_VERSION=5.3.1, Linux 4.9.7-x86_64-linode80 #2 SMP x86_64 GNU/Linux

Using any other combination (numeric file descriptor, here-string, etc)
seems to be fine.

This construction does not segfault on versions of bash I have available.