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Reported by @eveen:

Exact error message:

Overloading/Uniqueness error [nested.icl,20,Start]: "ctwo" uniqueness specification of instance conflicts with current application (? Int,? 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 (* ?a, * ?b) ?
where
	f ?None = (?None, ?None)
	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 :: * ?Int
	x = ?Just 1

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