Reported by @eveen:

Exact error message:

Overloading/Uniqueness error [nested.icl,20,Start]: "ctwo" uniqueness specification of instance conflicts with current application (Maybe Int,Maybe Int) Maybe

A minimal example:

module nested

import StdEnv
import StdMaybe

class cone v | ctwo (*v Int, *v Int) v

class ctwo a v
where
	f :: *(v *a) -> *a

instance ctwo (*Maybe a, *Maybe b) Maybe
where
	f Nothing = (Nothing, Nothing)
	f (Just x) = x

g :: (*v *(*v Int, *v Int)) -> *(*v Int, *v Int) | cone v
g x = f x

Start = g v
where
	x :: *Maybe Int
	x = Just 1

	v :: *Maybe *(*Maybe Int, *Maybe Int)
	v = Just (x, x)