[GHC] #14048: Data instances of kind Constraint

[GHC]

#14048: Data instances of kind Constraint
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        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):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Not to distract too much from the actual ticket (and perhaps this should
 be its own ticket), but it is rather disturbing that, as you briefly
 mentioned, `data Foo1 :: Constraint`/`data family Foo2 :: Constraint` is
 accepted today. Ultimately, it comes down to the fact that
 `isLiftedTypeKind` returns `True` for both `Type` //and// `Constraint`—see
 #14869—so GHC doesn't reject when the return kind is `Constraint`. I
 suppose that could be fixable by inserting an additional check for `not
 (isConstraintKind k)`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14048#comment:8>
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