[GHC] #13777: Poor error message around CUSKs

[GHC]

#13777: Poor error message around CUSKs
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc2
      Resolution:                    |             Keywords:  TypeInType,
                                     |  CUSKs
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4771
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ben Gamari <ben@…>):

 In [changeset:"ac91d07399207f4e22467bea3577cafd27a937d7/ghc"
 ac91d073/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="ac91d07399207f4e22467bea3577cafd27a937d7"
 Fix #13777 by improving the underdetermined CUSK error message

 The error message that GHC emits from underdetermined CUSKs
 is rather poor, since:

 1. It may print an empty list of user-written variables if there
     are none in the declaration.
 2. It may not mention any `forall`-bound, underdetermined
     variables in the result kind.

 To resolve these issues, this patch:

 1. Doesn't bother printing a herald about user-written
     variables if there are none.
 2. Prints the result kind to advertise any
     underdetermination it may exhibit.

 Test Plan: make test TEST=T13777

 Reviewers: goldfire, bgamari

 Reviewed By: goldfire

 Subscribers: rwbarton, thomie, carter

 GHC Trac Issues: #13777

 Differential Revision: https://phabricator.haskell.org/D4771
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13777#comment:2>
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