"opt_univ fell into a hole"

[Conal Elliott]

I'm getting the following error message from a GHC plugin I'm developing
(note GHC version):

    ghc-stage2: panic! (the 'impossible' happened)
      (GHC version 8.1.20160307 for x86_64-apple-darwin):
            opt_univ fell into a hole {aD1S}

I don't get this error when compiling without my plugin, so I may well be
violating a compiler invariant.

Shortly before the error, the plugin produced the following dictionary
expression, which does indeed contain a `UnivCo` with a `HoleProv`. The
plugin does sometimes generate `UnivCo`s but not with `HoleProv`.

    let {
      $dHasRep_aD1T :: HasRep (Vec 'Z s_aD1R[fuv:0])
      $dHasRep_aD1T = $fHasRepVec0 @ s_aD1R[fuv:0] } in
    let {
      $dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR :: HasRep (Vec 'Z (Key h_aCnh))
      $dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR =
        $dHasRep_aD1T
        `cast` ((HasRep
                   (Vec
                      <'Z>_N
                      (Sym U(hole:{aD1S}, Key h_aCnh,
s_aD1R[fuv:0])_N))_N)_R
                :: HasRep (Vec 'Z s_aD1R[fuv:0])
                   ~R# HasRep (Vec 'Z (Key h_aCnh))) } in
    $dHasRepZLVeczqZZZLKeyhZRZR_Ii5CR

Here, `Key` is a type family from `Data.Key` in the keys package, and `Vec`
is a GADT of statically length-indexed lists.

Suggestions?

Thanks, - Conal
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