T10570.hs:10:10: error:
    Illegal instance declaration for ‘ConsByIdx2 Int a Proxy cls’
      The coverage condition fails in class ‘ConsByIdx2’
        for functional dependency: ‘x -> m’
      Reason: lhs type ‘Int’ does not determine rhs type ‘Proxy’
      Un-determined variable: k
      (Use -fprint-explicit-kinds to see the kind variables in the types)
    In the instance declaration for ‘ConsByIdx2 Int a Proxy cls’