The following module module Bug where class C a b c | a b -> c where m :: a -> b -> c instance C Integer Integer Integer newtype T a = T a instance C a b c => C (T a) (T b) (T c) i :: T Integer i = undefined x = m (m i i) i generates the following error: $ hugs -98 Bug.hs __ __ __ __ ____ ___ _________________________________________ || || || || || || ||__ Hugs 98: Based on the Haskell 98 standard ||___|| ||__|| ||__|| __|| Copyright (c) 1994-2001 ||---|| ___|| World Wide Web: http://haskell.org/hugs || || Report bugs to: hugs-bugs@haskell.org || || Version: December 2001 _________________________________________ Hugs mode: Restart with command line option +98 for Haskell 98 mode Reading file "/usr/share/hugs/lib/Prelude.hs": Reading file "Bug.hs": Type checking ERROR "Bug.hs":16 - Unresolved top-level overloading *** Binding : x *** Outstanding context : C Integer Integer b If the definition of x is changed to x _ = m (m i i) i the module loads and x gets the type Bug> :t x x :: a -> T Integer So it looks like a monomorphism check is carried out too early in the first case. /M