[GHC] #12130: ghc: panic! (the 'impossible' happened): find_tycon Block []

[GHC]

#12130: ghc: panic! (the 'impossible' happened): find_tycon Block []
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        Reporter:  jeiea             |                Owner:  adamgundry
            Type:  bug               |               Status:  merge
        Priority:  high              |            Milestone:  8.0.2
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:
                                     |  DisambiguateRecordFields
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:  th/T12130
      Blocked By:                    |             Blocking:
 Related Tickets:  #11228            |  Differential Rev(s):  Phab:D2321
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"2f8cd14fe909a377b3e084a4f2ded83a0e6d44dd/ghc" 2f8cd14/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="2f8cd14fe909a377b3e084a4f2ded83a0e6d44dd"
 Narrow the use of record wildcards slightly

 In reviewing the fix to Trac #12130 I found the wild-card
 fill-in code for ".." notation in record constructions hard
 to understand.  It went to great contortions (including the
 find_tycon code) to allow
     data T = C { x, y :: Int }
     f x = C { .. }
 to expand to
     f x = C { x = x, y = y }
 where 'y' is an /imported function/!  That seems way over the top
 for what record wildcards are supposed to do.

 So I have narrowed the record-wildcard expansion to include only
 /locally-bound/ variables; i.e. not top level, and certainly not
 imported.

 I don't think anyone is using record wildcards in this bizarre way, so
 I don't expect any fallout. Even if there is, you can easily
 initialise fields with eponymous but imported values by hand.

 An intermediate position would be to allow /local/ top-level
 definitions.  But I doubt anyone is doing that either.

 Let's see if there's any fallout.  It's a local change, easy to
 revert, so I've just gone ahead to save everyone's time.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12130#comment:7>
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