[GHC] #11391: TypeError is fragile

[GHC]

#11391: TypeError is fragile
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        Reporter:  bgamari           |                Owner:  diatchki
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |             Keywords:
      Resolution:                    |  CustomTypeErrors
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9637             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by diatchki):

 OK, it was a pretty simple fix---I just pushed the correction to master.
 The problem was that when I was looking for `TypeError` in a type, I
 forgot that it may be "over applied", because it could be used at kinds
 like`Type -> Type`.

 I added a comment to `userTypeError_maybe` so we don't forget about this
 in the future.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11391#comment:7>
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