TH_repPatSigTVar.hs:8:64: error:
    • Not in scope: type variable ‘c’
    • In the Template Haskell quotation ''c
      In the untyped splice: $(sigE (varE 'undefined) (varT ''c))
      In the Template Haskell quotation
        [d| f = \ (_ :: Either a b)
                  -> $(sigE (varE 'undefined) (varT ''c)) |]