[GHC] #12131: Can't solve constraints with UndecidableSuperClasses but can infer kind (+ undesired order of kinds)

[GHC]

#12131: Can't solve constraints with UndecidableSuperClasses but can infer kind (+
undesired order of kinds)
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:                    |  UndecidableSuperClasses, TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11480 #12025     |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 1. `:kind` is much simpler than `:type`. `:kind` just reports the kind of
 the thing it sees. `:type`, on the other hand, instantiates, solves, and
 regeneralizes. It's the solver that's looping, so that's why only `:type`
 fails. (See #11376 for more info about why `:type` does this, and #11975
 for a proposed refinement of `:type` that will allow `:kind`-like
 behavior.)

 2. I think the algorithm that walks over the kind of a tycon and pulls out
 undeclared kind variables gets this backwards. I'm pretty sure that this
 has been reported elsewhere, but I can't find the ticket. In any case, I
 don't think anything deep is going on here.

 Let's not lose the big picture, though: the real problem in this ticket is
 that the solver loops, and that's what needs to be addressed. (I don't
 have further commentary on that point.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12131#comment:4>
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