[GHC] #12384: Type family not reduced, again

[GHC]

#12384: Type family not reduced, again
-------------------------------------+-------------------------------------
        Reporter:  kosmikus          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by alexvieth):

 #12381 and #11348 were all about the order in which instance declarations
 are checked, but this case seems to be something different. Here there's
 no choice but to check these declarations in the order that they're
 written: the type family, then the class, then the instance of that class.

 I suppose the type of `D ()` should be `(F a ~ Bool) => True` but as far
 as I know GHC can't handle such a thing.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12384#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler