GHC panic, `tcTyVarDetails` is missing a case

Compiling the attached module I get

$ ghc --interactive Tree.hs 
GHCi, version 8.1.20161030: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Tree             ( Tree.hs, interpreted )

Tree.hs:28:63: error:ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20161030 for x86_64-apple-darwin):
	tcTyVarDetails
  cobox0_a1eK :: (m_a1ej[sk:2] :: Peano)
                 ~#
                 ('S n_a1eJ[ssk:3] :: Peano)
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1076:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1080:37 in ghc:Outputable
        pprPanic, called at compiler/basicTypes/Var.hs:457:22 in ghc:Var

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

When switching the call to SBranchX in line 29 to a call to SBranch the program compiles. So I suspect that the problem is rooted in the extra constraint attached to constructor SBranchX.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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