Panic! make_exp (App _ (Coercion _)) when compiled with -fext-core

An attempt to compile the attached file with 7.6.2 (and 7.4.2) with "-O2 -fext-core" leads to:

ghc: panic! (the 'impossible' happened)
  (GHC version 7.6.2 for x86_64-unknown-linux):
    make_exp (App _ (Coercion _))

Edited Mar 09, 2019 by Ian Lynagh -

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information