[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 simonpj):

 While investigating #14270 I found that `SpecConstr` was generating rules
 with massive coercions, like
 {{{
 RULE "SC:foo"  f @(a  |>  <huge coercion expression>) arg1 arg2 = ...
 }}}
 Then I mis-read `Note [Matching in the presence of casts]` in `Unify`, and
 thought that we simply discarded casts. So I had a go at replacing casts
 in type patterns with a kind of placeholder `UnivCo`.

 I now think this is probably all wrong, because I still don't
 understanding of type patterns involving kind casts.  But rather than
 delete my changes entirely I'll attach them here for future reference.
 The starting point was a new form of `UnivCoProvenance` to use in type
 patterns; and a `TypePat` type in `Unify.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14119#comment:11>
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