match_co: needs more cases

[Evan Laforge]

I made some strictifying changes and started getting this msg from ghc:

match_co: needs more cases
    vector-0.10.0.1:Data.Vector.Generic.Mutable.MVector{tc r46}
      (Sym <(vector-0.10.0.1:Data.Vector.TFCo:R:MutableVector{tc r45})>)
      <main:Util.TimeVectorStorable.Sample{tc r4e} y{tv a4lK} [tv]>

It's not just TimeVectorStorable.Sample (which is indeed stored in
Data.Vectors), we also have some mysterious compiler-generated
symbols:

match_co: needs more cases
    vector-0.10.0.1:Data.Vector.Generic.Mutable.MVector{tc rQA}
      (Sym <(vector-0.10.0.1:Data.Vector.TFCo:R:MutableVector{tc rQz})>)
      <a{tv a6IH} [tv]>

I assume this is harmless, but I didn't see any other references to
this error on the web or on the ghc trac so maybe it's new?