pprPanic compiling `hashtables` package (GHC head)

Summary

ghc: panic! (the 'impossible' happened)
  (GHC version 8.11.0.20200620:
        urk! lookup local fingerprint
  EmptyElement
  [cESfjY :-> (Key, c95c9bd2232682a3ff3db6095ed5ddb6),
   iESflj :-> ($trModule, c3d17b474b3aa6c49cfa5f44523d660e),
   iESfll :-> ($trModule3, 6c39022df23a345128f3011a58ca4831),
   iESflm :-> ($trModule1, c9ad33973b0e938d4771a350975b57e1),
   iESflo :-> ($trModule2, 25989dc4f3bb4b8db88b64b50717e508),
   iESflp :-> ($trModule4, a5ea0043e4ed95827f3cc81e07d1e5cd),
   iEShIk :-> ($tc'EmptyElement, 79562e12a6a10f125f99790de6fae5ae),
   iEShIm :-> ($tcTombStone, b36edcacb6f137c85f02e04c068e68d6),
   iEShIp :-> ($tc'EmptyElement1, 24291eaccc11012b9a2224cd79c979d7),
   iEShIq :-> ($tc'EmptyElement2, 8ab16fee04b9ba027acabec4d15fec6a),
   iEShIr :-> ($tc'EmptyElement3, c6f99d035bf964ce09d6be780e1a3c02),
   iEShIs :-> ($tcTombStone1, 794a3fe1dfd8879bfa45ffeaac5fffec),
   iEShIt :-> ($tcTombStone2, a70e6297f3446513cfc2f6d032b2f0e2)]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1229:37 in ghc:GHC.Utils.Outputable
        pprPanic, called at compiler/GHC/Iface/Recomp.hs:811:37 in ghc:GHC.Iface.Recomp

Steps to reproduce

Minimal reproducer: Repro.hs. Compile with -O2.

Expected behavior

No panic

Environment

GHC version used: Head commit d4a0be75

Optional:

Operating System: FreeBSD 12
System Architecture: x86_64

Edited Jun 21, 2020 by vdukhovni

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