Re: [GHC] #12102: “Constraints in kinds” illegal family application in instance (+ documentation issues?)

[GHC]

#12102: “Constraints in kinds” illegal family application in instance (+
documentation issues?)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13780            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 In GHC HEAD (as of commit 2257a86daa72db382eb927df12a718669d5491f8), you
 get the following error when compiling the original program:

 {{{
 $ inplace/bin/ghc-stage2 --interactive ../Bug.hs
 GHCi, version 8.7.20181129: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( ../Bug.hs, interpreted )

 ../Bug.hs:15:1: error:
     • Illegal constraint in a kind: forall a.
                                     (IsTypeLit a ~ 'True) =>
                                     a -> *
     • In the data type declaration for ‘T’
    |
 15 | data T :: forall a. (IsTypeLit a ~ 'True) => a -> * where
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
 }}}

 Which would suggest that this issue is now moot.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12102#comment:13>
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