Discrepancies between derived Show instances and `structuralShowsPrec`

In certain cases, the output of structuralShowsPrec will differ from what deriving Show produces. Here is a test case which illustrates this:

{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Data.Parameterized.TH.GADT

data T
  = MkRec { unRec :: Int }
  | Int :@@: Int
  deriving Show

$(return [])

puShowT :: T -> String
puShowT t = $(structuralShowsPrec [t| T |]) 0 t ""

main :: IO ()
main = do
  let r   = MkRec 42
      inf = 42 :@@: 42
  putStrLn $ "MkRec (derived):\t\t" ++ show r
  putStrLn $ "MkRec (parameterized-utils):\t" ++ puShowT r
  putStrLn $ "(:@@:) (derived):\t\t" ++ show inf
  putStrLn $ "(:@@:) (parameterized-utils):\t" ++ puShowT inf

This will output:

MkRec (derived):                MkRec {unRec = 42}
MkRec (parameterized-utils):    MkRec 42
(:@@:) (derived):               42 :@@: 42
(:@@:) (parameterized-utils):   :@@: 42 42

This shows that structuralShowsPrec ignores record and infix data constructors, which deriving Show is aware of.

Start with the supplied Haskell reproduction and inspect the implementation of structuralShowsPrec. Compare its output with derived Show for the record and infix constructors shown in the example. Done means both forms use the same formatting as deriving Show, with regression coverage for these cases.