[GHC] #16140: Cannot create type synonym for quantified constraint without ImpredicativeTypes

[GHC]

#16140: Cannot create type synonym for quantified constraint without
ImpredicativeTypes
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        Reporter:  Ashley Yakeley    |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints,
                                     |  ImpredicativeTypes
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:                    |
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Comment (by simonpj):

 > Do you think we'd be better off not calling checkValidType here?

 Yes!  If we are going to have `checkTySynRhs` at all, then we should do
 all the work there.

 This is the commit that added `checkTySynRhs`.
 {{{
 commit 35c9de7ca053eda472cb446c53bcd2007bfd8394
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Sat Jun 11 23:56:42 2016 +0100

     Move the constraint-kind validity check

     For type synonyms, we need to check that if the RHS has
     kind Constraint, then we have -XConstraintKinds.  For
     some reason this was done in checkValidType, but it makes
     more sense to do it in checkValidTyCon.

     I can't remember quite why I made this change; maybe it fixes
     a Trac ticket, but if so I forget which.  But it's a modest
     improvement anyway.
 }}}
 But calling both makes no sense.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:19>
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