[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:
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 goldfire):

 This is an interesting question. When written to the left of an `=>`, the
 parentheses delimit a list that is inlined. However, in a type synonym,
 you're really building a tuple. So GHC's behavior here is just a
 manifestation of this internal design... but the design shouldn't leak
 this way.

 I would classify it as a bug, yes. It's unclear to me how to solve it,
 though, especially because GHC struggles to tell constraint tuples apart
 from normal ones.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16140#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler