roles are more restrictive (but not for newtype instance)

VectorDataFamily.hs

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeFamilies #-}
import qualified Data.Vector as V
import qualified Data.Vector.Mutable as VM
import qualified Data.Vector.Generic as G
import Data.Coerce
import Control.Monad.Primitive
import Foreign.C

data family MVec (v :: * -> *) :: * -> * -> *

newtype instance MVec V.Vector a b = VM_Vector (VM.MVector a b)

coerce_was_bad :: MVec V.Vector RealWorld Double -> MVec V.Vector RealWorld CDouble
coerce_was_bad = coerce
{- this error only happens if the MVec instance was a Data instance:

    Could not coerce from ‘MVec V.Vector RealWorld Double’ to ‘MVec
                                                                 V.Vector RealWorld CDouble’
      because the third type argument of ‘MVec’ has role Nominal,
      but the arguments ‘Double’ and ‘CDouble’ differ
      arising from a use of ‘coerce’
    In the expression: coerce
    In an equation for ‘coerce_bad’: coerce_bad = coerce
-}


coerce_good :: G.Mutable V.Vector RealWorld Double -> G.Mutable V.Vector RealWorld CDouble
coerce_good = coerce