I just noticed that if you derive Real for a Ratio.Rational newtype, you get a warning: {-# LANGUAGE GeneralizedNewtypeDeriving #-} import qualified Data.Ratio as Ratio newtype X = X Ratio.Rational deriving (Eq, Ord, Num, Real) You get a warning: <no location info>: Warning: Call of toRational :: Rational -> Rational can probably be omitted (Use -fno-warn-identities to suppress this message) I suppose the automatic Real instance uses toRational to, um, define toRational? In any case, the <no location info> is certainly a bug, but it looks like ghc 8.0.2 has fixed it. Aside from that, it would be nice to automatically suppress warn-identities and unused code warnings in general for automatically generated code that we don't really have any control over and can't see in the source. It's easily worked around with OPTIONS_GHC to turn off the warning, but it's confusing the first time you see it.