Unexpected results with Read/Show

In the following simplified example, Foo and U correspond to GADTs that GHC will not derive Read/Show for. I attempted to work around that by using newtypes for each GADT constructor, and letting GHC derive the Show/Read instances for those instead. However, I get a runtime error (Prelude.read: no parse) on the second print statement in main:

{-# LANGUAGE FlexibleContexts, FlexibleInstances, GADTs, ScopedTypeVariables #-}

import Text.Read (Read(readPrec))

newtype Bar r = Bar r deriving (Show, Read)
newtype Foo r = Foo (Bar r)
-- use the GHC-derived Show/Read for Bar
instance (Show r) => Show (Foo r) where
  show (Foo x) = show x
instance (Read r) => Read (Foo r) where
  readPrec = Foo <$> readPrec

data U t rep r where
  U1  :: t r -> U t Int r
  U2 :: t r -> U t Char r
-- use the Read/Show instances for U1Wrap and U2Wrap
newtype U1Wrap t r = U1Wrap {unU1Wrap :: t r} deriving (Show, Read)
newtype U2Wrap t r = U2Wrap (t r) deriving (Show, Read)
instance (Read (t r)) => Read (U t Int r) where
  readPrec = (U1 . unU1Wrap) <$> readPrec
instance (Read (U2Wrap t r)) => Read (U t Char r) where
  readPrec = do
    x <- readPrec
    return $ case x of
      (U2Wrap y) -> U2 y
instance (Show (t r)) => Show (U t Int r) where
  show (U1 x) = show $ U1Wrap x
instance (Show (t r)) => Show (U t Char r) where
  show (U2 x) = show (U2Wrap x :: U2Wrap t r)

main :: IO ()
main = do
  let x = U1 $ Foo $ Bar 3
      y = U2 $ Foo $ Bar 3
  print $ show (read (show x) `asTypeOf` x)
  print $ show (read (show y) `asTypeOf` y)

Someone mentioned that I should define showsPrec rather than show, but these are listed as alternatives in the docs.

It's not clear to me if GHCs derived instances are invalid, or if I'm doing something illegal. In the latter case, the docs need some improvement.

(Verified this behavior in 7.10.2 and HEAD.)

Trac metadata

Trac field	Value
Version	7.10.3
Type	Bug
TypeOfFailure	IncorrectResultAtRuntime
Priority	normal
Resolution	Unresolved
Component	Compiler
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