[GHC] #9574: GHC Panic: No Skolem Info

[GHC]

#9574: GHC Panic: No Skolem Info
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              Reporter:  ian_mi      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.9
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"171101beca39befde191baff5027c417bcc709ee/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="171101beca39befde191baff5027c417bcc709ee"
 Kind variables in RHS of an associated type instances should be bound on
 LHS

 This patche fixes Trac #9574.

 The previous Note [Renaming associated types] in RnTypes appears to me to
 be wrong;
 it confused class and instance declarations.

 I have:

  * Treated kind and type variables uniformly. Both must be bound on the
 LHS
    of an associated type instance.  Eg
        instance C ('KProxy :: KProxy o) where
           type F 'KProxy = NatTr (Proxy :: o -> *)
    is illegal because 'o' is not bound on the LHS of the instance.

  * Moved the Note to RnSource and fixed it up

 This improves the error message from T7938. However it made the code in
 T6118 incorrect. We had:
   instance SingE (a :: Maybe k) where
     type Demote a = Maybe (Demote (Any :: k))
 and that is now rejected, rightly I think.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9574#comment:7>
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