In a record-update construct:ghc-stage2: panic! (the 'impossible' happened)

While compiling Repro.hs with HEAD I get this:

$ ~/local/ghc-inaccessible-right-side/inplace/bin/ghc-stage2 Repro.hs
[1 of 1] Compiling Repro            ( Repro.hs, Repro.o )

Repro.hs:22:35: warning: [-Woverlapping-patterns]
    Pattern match has inaccessible right hand side
    In a record-update construct:ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20161209 for x86_64-unknown-linux):
        unused

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

It doesn't reproduce on 7.10.2. I'm 90% sure it reproduces on 8.0.2 (I have a branch with some unrelated patches).

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Trac field	Value
Version
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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