GHC said to report a bug

I tried to run

{-# LANGUAGE Rank2Types, PartialTypeSignatures #-}
import Control.Monad
newtype Search b = Search {optimum :: forall a. Ord a => (b -> a) -> b}

instance Monad Search where
  return a = Search $ const a
  Search ma >>= f = Search $ \p ->
    optimum (f (ma (\a -> p (optimum (f a) p)))) p

instance Functor Search where
  fmap = liftM

instance Applicative Search where
  pure = return
  (<*>) = ap

pair a b = Search $ \p -> if p a >= p b then a else b
toList s = igo [] where
    igo ls = let x = optimum s (\x' -> x' `notElem` ls) in if x `elem` ls then ls else igo (x:ls)

cantor :: Search (Integer -> Bool)
cantor = let
    igo :: _
    igo p n = q n (optimum cantor $ q n) where
        q n a = p undefined
    in Search igo

main = return ()

When run, it said

source_file.hs:24:5:
    No instance for (Ord a)
    When checking that ‘igo’ has the specified type
      igo :: forall t a t1. (t -> a) -> t1 -> a
    Probable cause: the inferred type is ambiguous
    In the expression:
      let
        igo :: _
        igo p n
          = q n (optimum cantor $ q n)
          where
              q n a = ...
      in Search igo
    In an equation for ‘cantor’:
        cantor
          = let
              igo :: _
              igo p n
                = q n (optimum cantor $ q n)
                where
                    ...
            in Search igo

source_file.hs:26:15:
    Couldn't match type ‘t’ with ‘Integer -> Bool’
      ‘t’ is untouchable
        inside the constraints (Ord a1)
        bound by a type expected by the context:
                   Ord a1 => ((Integer -> Bool) -> a1) -> Integer -> Bool
        at source_file.hs:26:8-17ghc: panic! (the 'impossible' happened)
  (GHC version 7.10.3 for x86_64-unknown-linux):
	No skolem info: t_aU3[sk]

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

Since it said to report a bug, I am.

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture