[GHC] #8368: Panic in rejigConRes

[GHC]

#8368: Panic in rejigConRes
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        Reporter:  goldfire          |            Owner:  goldfire
            Type:  bug               |           Status:  patch
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by Krzysztof Gogolewski <krz.gogolewski@?>):

 In [changeset:0c7d2d7507b9e4dca24c159c0bfcce9de7b233c4/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="0c7d2d7507b9e4dca24c159c0bfcce9de7b233c4"
 Fix Trac #8368.

 Two different fixes were necessary here. First, we need to fail eagerly
 in kcConDecl, to prevent the return-type error in tcConDecl from firing
 twice. (This wasn't caught earlier because of the eager fail in the
 datatype kind-checking code -- which isn't used for data instances!)
 We also must check again in tcDataFamInstDecl, because it's possible for
 a data instance return type to have the right head but the wrong body
 (i.e., doesn't conform to the data instance type patterns). This check
 is only possible *after* desugaring from HsType to Type, so it can't be
 done in tcConRes with the first check.

 This is documented in a comment at check_valid_data_con, a local
 function within tcDataFamInstDecl.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8368#comment:4>
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