The latest installment in "Ryan finds minor bugs in -ddump-splices". Take this program:

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

$([d| f :: Bool -> Bool
      f x = case x of
              (True :: Bool) -> True
              (False :: Bool) -> False

      g :: Bool -> Bool
      g x = (case x of
               True -> True
               False -> False) :: Bool
    |])

Compiling this gives:

$ /opt/ghc/8.2.2/bin/ghci Bug.hs
GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(6,3)-(15,6): Splicing declarations
    [d| f_a1sB :: Bool -> Bool
        f_a1sB x_a1sD
          = case x_a1sD of
              (True :: Bool) -> True
              (False :: Bool) -> False
        g_a1sC :: Bool -> Bool
        g_a1sC x_a1sE
          = (case x_a1sE of
               True -> True
               False -> False) ::
              Bool |]
  ======>
    f_a49Z :: Bool -> Bool
    f_a49Z x_a4a0
      = case x_a4a0 of
          True :: Bool -> True
          False :: Bool -> False
    g_a49Y :: Bool -> Bool
    g_a49Y x_a4a1
      = case x_a4a1 of
          True -> True
          False -> False ::
          Bool

Neither the -ddump-splices output for f nor g parse are legal Haskell:

• In f, GHC fails to parenthesize the pattern signatures True :: Bool and False :: Bool.
• In g, GHC fails to parenthesize the case expression which has an explicit Bool signature.