[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):

 This has nothing to do with this ticket, but rather #16059. Moreover, this
 is expected behavior—the fix for #16059 revealed an illegal use of
 impredicativity in `Cabal-2.4.1.0` that previously went unnoticed (see the
 discussion starting at
 https://ghc.haskell.org/trac/ghc/ticket/16059#comment:14). The current
 version of `Cabal` that is bundled with GHC HEAD, `Cabal-2.5.0.0`, fixes
 this issue.

 **tl;dr** You want to use `Cabal-2.5.0.0`, not `Cabal-2.4.1.0`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:21>
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