Segfault in GHC running check-exact

This is a marker for a segfault I am seeing on my ghc-exactprint update, which I have captured in branch wip/az/T21355-exactprint-update-segfault

Demonstrates segfault in ghci

In the utils/check-exact directory do

../../_build/stage1/bin/ghc --interactive
:l Main.hs
:main

It segfaults.

Do

../../_build/stage1/bin/ghc --interactive
:set -fobject-code
:l Main.hs
:main

It runs successfully (logging a lot of stuff).

The two logs are different, the one just before the segfault returns
junk for an AddEpAnn value.
See https://gist.github.com/alanz/9b944a59dbaa04f5d502fb5244d83270
for some detail

It is from a work in progress branch, and needs to be distilled to something useful. But I have a workaround by using -fobject-code so will proceed that way for a little longer. And will revisit some time, if no-one else does.

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