[GHC] #15799: GHC panic (and warnings)

[GHC]

#15799: GHC panic (and warnings)
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12045            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Discussion this morning essentially agreed with my "first instinct" above.
 But with a new spin on things: The `tcm_smart` setting on `TyCoMapper` is
 really about what to do about `Refl` casts. Do we preserve them? Discard
 them? Or, in our new mode, discard some of them? Previously, the "dumb"
 mode meant to preserve all casts. This is important for `Note [The well-
 kinded type invariant]` (where we sometimes use reflexive casts to make a
 type's kind looked zonked even when the type is not yet zonked). The
 "smart" mode dropped all reflexive casts. Now, we want a third mode that
 drops all casts that look reflexive to the type checker.

 By elucidating what we really mean by "smart", having a third setting does
 not look so unusual, after all.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15799#comment:4>
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