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

[GHC]

#16140: Cannot create type synonym for quantified constraint without
ImpredicativeTypes
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Replying to [comment:15 simonpj]:
 > * `checkValidType` uses `check_type`, not `check_pred_ty`.

 I'm confused—didn't you say (in comment:3) that `checkValidType` should
 use `check_pred_ty` for `Constraint`-kinded things? (Now that I think
 about it, this is exactly why these errors are popping up—VTA simply
 didn't go down the `check_pred_ty` path before.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler