[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 RyanGlScott):

 The good news is that #16059 has been fixed. The downside is that I just
 discovered that the only reason that I managed to get the program in this
 ticket to work (after implementing the suggestion in comment:3) is
 //because// of #16059! That is to say, when I try implementing comment:3
 now, GHC's improved type validity-checker now rejects the program in this
 ticket. Bummer.

 It looks like calling `check_pred_ty` isn't enough—there must be something
 deeper that needs to be changed.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:10>
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