Compilation panic! (the 'impossible' happened)

See attached zip for entire project causing error. I'd imagine it's pretty reproducible, just call stack build. I'm running standard lts-12.8 and it's just two small files that don't do anything yet! Also included is panicmessage.txt showing the exact output, crucially:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-unknown-linux):
	piResultTy
  *
  v_XabT
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:947:35 in ghc:Type

Oh and sysinfo

uname -a
Linux @@@ 4.16.11-100.fc26.x86_64 #1 SMP Tue May 22 20:02:12 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Linux
Architecture	x86_64 (amd64)