[GHC] #14111: strange error when using data families with levity polymorphism and unboxed sums and data families

[GHC]

#14111: strange error when using data families with levity polymorphism and unboxed
sums and data families
-------------------------------------+-------------------------------------
        Reporter:  carter            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:  13737             |             Blocking:
 Related Tickets:  #14457            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Phab:D4974 purports to "get rid of the whole knot-tying business"... or so
 I thought. Actually, I don't think even Phab:D4974 would be sufficient to
 unblock this. The problem is that even with that patch, `res_tmpl` in
 comment:14 is //still// knot-tied (since it mentions the knot-tied
 `rep_tycon`). This means calling `unifyType` on it is doomed to loop
 infinitely.

 Alas, https://phabricator.haskell.org/D4974#137148 suggests that
 `rep_tycon` is destined to be knot-tied, so I'm not sure how this bug
 could ever be fixed.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14111#comment:19>
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