Daniel Fischer wrote:
No, the instance head is just a type variable, not a type constructor applied to type variables
Oops, you're right. GHC was telling the truth, I should have paid closer attention! Fixing my minimal example, I get: {-# LANGUAGE MultiParamTypeClasses, UndecidableInstances #-} module ClassContextBug where class A a oops class B b data D d = D d instance A a oops => B (D a) and now GHC compiles it happily. So it's the derived Read instance in this context that is causing the problem. Here is a slightly smaller test case that triggers the bug: {-# LANGUAGE MultiParamTypeClasses, UndecidableInstances #-} module Bug where class A a oops data D d = D d instance A a oops => Read (D a) data E e = E (D e) deriving Read Thanks, Yitz