Extraneous parentheses required to parse kind signature on the RHS of a type synonym

After D5173, the restrictions about where kind signatures can appear in the source were significantly relaxed so that things like:

type family F where
  F = Int :: Type

Are now allowed. However, there appears to be one case that was missed in that patch: the right-hand sides of type synonyms. For instance, I would expect the following to parse:

{-# LANGUAGE KindSignatures #-}
module Bug where

import Data.Kind

type F = Int :: Type

However, even GHC HEAD still refuses to parse this:

$ /opt/ghc/head/bin/ghci Bug.hs
GHCi, version 8.7.20181015: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:6:14: error: parse error on input ‘::’
  |
6 | type F = Int :: Type
  |              ^^

Luckily, I don't think this will be too difficult to fix, since all that needs to be done is to update the parser production for type synonyms to use something like ktype instead of ctype. Patch incoming.

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Parser)
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