[GHC] #13674: GHC doesn't discharge heterogeneous equality constraint when it ought to

[GHC]

#13674: GHC doesn't discharge heterogeneous equality constraint when it ought to
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 The original program is wrong, but the error message also is wrong.
 Because `Lcm` is a nullary type family, the constraint `m ~ Lcm m m` is
 morally equivalent to `m ~ Either m m`. This constraint is an occurs-check
 failure and is surely going to lead to an error. It seems that the error-
 reporting code is mischaracterizing it, though.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13674#comment:5>
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