[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:
                                     |  InjectiveFamilies
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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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"d6216443c61cee94d8ffc31ca8510a534d9406b9/ghc"
 d6216443/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="d6216443c61cee94d8ffc31ca8510a534d9406b9"
 Fix an infinite loop in niFixTCvSubst

 Trac #14164 made GHC loop, a pretty serious error. It turned
 out that Unify.niFixTCvSubst was looping forever, because we
 had a substitution like
     a :-> ....(b :: (c :: d))....
     d :-> ...
 We correctly recognised that d was free in the range of the
 substitution, but then failed to apply it "deeeply enough"
 to the range of the substiuttion, so d was /still/ free in
 the range, and we kept on going.

 Trac #9106 was caused by a similar problem, but alas my
 fix to Trac #9106 was inadequate when the offending type
 variable is more deeply buried.  Urk.

 This time I think I've fixed it!  It's much more subtle
 than I though, and it took most of a long train journey
 to figure it out.  I wrote a long note to explain:
 Note [Finding the substitution fixpoint]
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14164#comment:5>
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