[GHC] #15781: Extraneous parentheses required to parse kind signature on the RHS of a type synonym

19 Oct 2018

      #15781: Extraneous parentheses required to parse kind signature on the RHS of a
type synonym
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.7
  (Parser)                           |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 After Phab:D5173, the restrictions about where kind signatures can appear
 in the source were significantly relaxed so that things like:

 {{{#!hs
 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:

 {{{#!hs
 {-# 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.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15781
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler