[GHC] #14164: GHC hangs on type family dependency

[GHC]

#14164: GHC hangs on type family dependency
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * cc: RyanGlScott (added)


Comment:

 I traced this down to a diverging call to
 [http://git.haskell.org/ghc.git/blob/682e8e6e01cd9c96378692656649094ad44c35a7:/compiler/types/FamInstEnv.hs#l576
 tcUnifyTyWithTFs] in `FamInstEnvs.injectiveBranches`. I don't know //why//
 it's diverging, but I do know that its arguments `rhs1` and `rhs2` are
 (according to `pprTrace`):

 {{{
 (rhs1) y_a1tT : UpdateWith ys2_a1tU xs_a1tV rest_a1tW
 (rhs2) y_a1tR : ys_a1tS
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14164#comment:2>
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