[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):

 > TypeApplications now require language extensions that they didn't
 before.

 This one is more mysterious to me.
 * The arguments to a VTA is checked with `checkValidType` (see
 `TcHsType.tcHsTypeApp`).
 * `checkValidType` uses `check_type`, not `check_pred_ty`.

 So how come we end up in `check_pred_ty`?  I don't think we want to; e.g.
 `Eq Int` is a perfectly fine instantiation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:15>
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