Read1/Show1 instances for Complex use incorrect precedence

The Read1 and Show1 instances for Complex handle precedence in a way that is inconsistent with Complex's Read and Show instances. This is best illustrated by example:

module Main (main) where

import Data.Complex (Complex(..))
import Data.Functor.Classes (readPrec1, showsPrec1)
import Text.ParserCombinators.ReadPrec (readPrec_to_S)
import Text.Read (Read(..))

comp :: Complex Int
comp = 1 :+ 1

compareInstances :: Int -> IO ()
compareInstances p = do
  let precBanner = " (at precedence " ++ show p ++ ")"
  putStrLn $ "Read vs. Read1" ++ precBanner
  print (readPrec_to_S readPrec  p "1 :+ 1" :: [(Complex Int, String)])
  print (readPrec_to_S readPrec1 p "1 :+ 1" :: [(Complex Int, String)])
  putStrLn ""
  putStrLn $ "Show vs. Show1" ++ precBanner
  putStrLn $ showsPrec  p comp ""
  putStrLn $ showsPrec1 p comp ""
  putStrLn ""

main :: IO ()
main = do
  compareInstances 6
  compareInstances 7

If you run this with GHC 9.2.1-alpha1, it will produce the following output:

Read vs. Read1 (at precedence 6)
[(1 :+ 1,"")]
[(1 :+ 1,"")]

Show vs. Show1 (at precedence 6)
1 :+ 1
1 :+ 1

Read vs. Read1 (at precedence 7)
[]
[(1 :+ 1,"")]

Show vs. Show1 (at precedence 7)
(1 :+ 1)
1 :+ 1

At precedence 6, the Read1/Show1 instances behave analogously to the Read/Show instances, as expected. At precedence 7, however, they produce different results! In particular:

• The Read instance will fail to parse "1 :+ 1" at precedence 7, since it should be surrounded by parentheses at that precedence due to :+'s fixity of 6. The Read1 instance, however, will incorrectly parse the same string without the required parentheses.
• The Show instance will print 1 :+ 1 with surrounding parentheses at precedence 7, again because of :+'s fixity of 6. The Show1 instance, however, will incorrectly omit these parentheses.

Both issues are caused by the Read1/Show1 instances incorrectly assuming that :+ has a fixity of 9, rather than its actual fixity of 6. Because the Read and Show instances are derived, it automatically uses the correct fixity, but the handwritten Read1/Show1 instances do not mirror this. This could be fixed as simply as:

diff --git a/libraries/base/Data/Functor/Classes.hs b/libraries/base/Data/Functor/Classes.hs
index 6a0d008982..d672c340d7 100644
--- a/libraries/base/Data/Functor/Classes.hs
+++ b/libraries/base/Data/Functor/Classes.hs
@@ -848,11 +848,13 @@ instance Eq1 Complex where
 -- [(2 % 3 :+ 3 % 4,"")]
 --
 instance Read1 Complex where
-    liftReadPrec rp _  = parens $ prec 9 $ do
+    liftReadPrec rp _  = parens $ prec complexPrec $ do
         x <- step rp
         expectP (Symbol ":+")
         y <- step rp
         return (x :+ y)
+      where
+        complexPrec = 6
 
     liftReadListPrec = liftReadListPrecDefault
     liftReadList     = liftReadListDefault
@@ -863,8 +865,10 @@ instance Read1 Complex where
 -- "2 :+ 3"
 --
 instance Show1 Complex where
-    liftShowsPrec sp _ d (x :+ y) = showParen (d >= 10) $
-        sp 10 x . showString " :+ " . sp 10 y
+    liftShowsPrec sp _ d (x :+ y) = showParen (d > complexPrec) $
+        sp (complexPrec+1) x . showString " :+ " . sp (complexPrec+1) y
+      where
+        complexPrec = 6
 
 -- Building blocks

These instances were introduced in GHC 9.2, so there's still time to fix them before the final 9.2.1 release. Patch incoming.