Core lint error in simplifier when compiling Rules1 with -O -dcore-lint

Running make TEST=Rules1 WAY=optasm results in a core lint error (see attachment) with ghc-8.0.1-rc2.

This is the code:

{-# LANGUAGE TypeFamilies #-}

module Rules1 where

class C a where
  data T a

instance (C a, C b) => C (a,b) where
  data T (a,b) = TPair (T a) (T b)

mapT :: (C a, C b) => (a -> b) -> T a -> T b
{-# NOINLINE mapT #-}  -- Otherwwise we get a warning from the rule
mapT = undefined

zipT :: (C a, C b) => T a -> T b -> T (a,b)
{-# NOINLINE [1] zipT #-}
zipT = undefined

{-# RULES

"zipT/mapT" forall f x y.
  zipT (mapT f x) y = mapT (\(x,y) -> (f x, y)) (zipT x y)

 #-}

$ ghc-8.0.1 -O Rules1.hs -dcore-lint

This is a regression from 7.10.3, and core lint errors are bad, so setting priority to highest.