[GHC] #14048: Data instances of kind Constraint

[GHC]

#14048: Data instances of kind Constraint
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12369            |  Differential Rev(s):  Phab:D4479
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"f748c52997f61a9f58eccbf4b8df0a0c8c6887e5/ghc"
 f748c529/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="f748c52997f61a9f58eccbf4b8df0a0c8c6887e5"
 Don't permit data types with return kind Constraint

 Previously, GHC allowed all of the following:

 ```lang=haskell
 data Foo1 :: Constraint
 data family Foo2 :: Constraint
 data family Foo3 :: k
 data instance Foo3 :: Constraint
 ```

 Yikes! This is because GHC was confusing `Type` with `Constraint`
 due to careless use of the `isLiftedTypeKind` function. To respect
 this distinction, I swapped `isLiftedTypeKind` out for
 `tcIsStarKind`—which does respect this distinction—in the right
 places.

 Test Plan: make test TEST="T14048a T14048b T14048c"

 Reviewers: bgamari

 Reviewed By: bgamari

 Subscribers: goldfire, rwbarton, thomie, carter

 GHC Trac Issues: #14048

 Differential Revision: https://phabricator.haskell.org/D4479
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14048#comment:10>
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