[GHC] #14119: Refactor type patterns

[GHC]

#14119: Refactor type patterns
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        Reporter:  goldfire          |                Owner:  goldfire
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12564, 13910,
                                     |  13938, 14038
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"9218ea60faa744803fb41066aedd1da8f1bd9185/ghc"
 9218ea60/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="9218ea60faa744803fb41066aedd1da8f1bd9185"
 Interim fix for a nasty type-matching bug

 The type matcher in types/Unify.hs was producing a substitution
 that had variables from the template in its range.  Yikes!

 This patch, documented in Note [Matching in the presence of casts],
 is an interm fix.  Richard and I don't like it much, and are
 pondering a better solution (Trac #14119).

 All this came up in investigating Trac #13910. Alas this patch
 doesn't fix #13910, which still has ASSERT failures, so I have
 not yet added a test.  But the patch does fix a definite bug.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14119#comment:10>
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