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

[GHC]

#8978: Type synonyms in class associated types behave strangely
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        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:
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"b8132a9d2fdb93c5d30107b1d531dd73ac27b262/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="b8132a9d2fdb93c5d30107b1d531dd73ac27b262"
 Fix egregious blunder in the type flattener

 In tidying up the flattener I introduced an error that no
 regression test caught, giving rise to Trac #8978, #8979.
 It shows up if you have a type synonym whose RHS mentions
 type functions, such sas
      type family F a
      type T a = (F a, a)   -- This synonym isn't properly flattened

 The fix is easy, but sadly the bug is in the released GHC 7.8.1
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8978#comment:11>
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