[GHC] #12026: Pattern match failure in RnNames.hs

[GHC]

#12026: Pattern match failure in RnNames.hs
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        Reporter:  davean            |                Owner:  mpickering
            Type:  bug               |               Status:  patch
        Priority:  highest           |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2181
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"53f26f5a45f146e1cc988bbcf76a362c877beaa2/ghc" 53f26f5/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="53f26f5a45f146e1cc988bbcf76a362c877beaa2"
 Forbid variables to be parents in import lists.

 In the long discussion on #11432, it was decided that when a type
 constructor is parsed as a variable ((--.->) is one example) then in
 order to export the type constructor then the user should be required to
 use the ExplicitNamespaces keyword.

 This was implemented in quite an indirect manner in the renamer. It
 is much more direct to enforce this in the parser at the expense of
 slighty worse error messages. Further to this, the check in the
 renamer was actually slightly wrong. If the variable was in scope
 then no error was raised, this was causing panics, see #12026 for an
 example.

 Reviewers: austin, bgamari

 Subscribers: davean, skvadrik, thomie

 Differential Revision: https://phabricator.haskell.org/D2181

 GHC Trac Issues: #12026
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12026#comment:6>
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