[GHC] #8978: Type synonyms in class associated types behave strangely

[GHC]

#8978: Type synonyms in class associated types behave strangely
----------------------------------------------+----------------------------
        Reporter:  joelteon                   |            Owner:
            Type:  bug                        |           Status:  new
        Priority:  normal                     |        Milestone:
       Component:  Compiler                   |          Version:  7.8.1
      Resolution:                             |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
----------------------------------------------+----------------------------

Comment (by simonpj):

 Ugh.  This is my fault; an egregious blunder in a (cool) clean-up of the
 type flattener. It's easy to fix (patch coming) but the bug is in the
 released 7.8.1.  Will fix in 7.8.2.  It just shows, you never have enough
 regression tests!

 It affects any type synonym whose RHS mentions a type family.  (Data
 families are fine.) Such as `Syn` above, or `H` in #8979.

 Richard's workaround is good; use a type family instead.

 Sorry about this.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8978#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler