[GHC] #11970: Simplify Parent

[GHC]

#11970: Simplify Parent
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           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 This commit
 {{{
 commit 96621b1b4979f449e873513e9de8d806257c9493
     Associate pattern synonyms with types in module exports
 }}}
 added `PatternSynonym` to `RdrName.Parent`.  That's a pretty heavy hammer.
 It forces us to add `IsPatSyn` to `AvailInfo`, including binary
 serialisation etc.

 And I think it's needed in precisely one place, namely
 `RnNames.findPatSyns`.  Right?

 The caller of `findPatSyns` and the associated `lookupChildren` is jolly
 obscure and I don't fully understand it.  I **think** the issue is this:

 * I want to allow an export item `T( K )`, where `K` is a pattern synonym,
 even though it's not really (yet) associated with `T`.

 I think the goal of this code was to reject the `K` if it's a data
 constructor.

 But we also have a **further** check in `tcExports` to check that `K` has
 the right type.

 Surely it'd be better to nuke this `PatternSynonym` and `IsPatSyn` stuff?
 Just use`isDataOcc` in the export lookup part.  Then the `tcExport` check
 will object if you write an export item like `Either( Just )`.

 Fewer moving parts!

 Matthew, does this make sense?  Might you look at it?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11970>
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