ghc: panic! (the 'impossible' happened)

The attached program elicits the following error from GHC 7.10.2:

    Couldn't match type ‘a’ with ‘(t0, t1, t2)’
      ‘a’ is untouchable
        inside the constraints ()
        bound by the inferred type of
                 updateKeytab :: FilePath -> Handle -> IO ()
        at /u/res/g/src/remctl-tools/foo.hs:(58,1)-(69,26)ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.2 for x86_64-unknown-linux):
	No skolem info: a_akOb[sk]

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

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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