ghc: panic: tyThingTyCon Identifier ‘$fStencil:.a(,,,,)7’

Has anyone seen something like this?

<no location info>:
    ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.1.20150519 for x86_64-unknown-linux):
	tyThingTyCon Identifier ‘$fStencil:.a(,,,,)7’

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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