[GHC] #12950: unnecessarily complicated left-hand side causing RULE left-hand side too complicated to desugar

[GHC]

#12950: unnecessarily complicated left-hand side causing RULE left-hand side too
complicated to desugar
-------------------------------------+-------------------------------------
        Reporter:  nfrisby           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.2
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
  #10555,#12074,#12649               |
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by nfrisby):

 * related:  #10555,#12074 => #10555,#12074,#12649


Comment:

 The proposal in #12649 might involve an "alternate" code path in the type-
 checker that side-steps this issue: if a SPECIALIZE pragma could be
 declared more directly ("set `a` to `Int`"), then `decomposeRuleLhs` (the
 function that emits "RULE left-hand side too complicated to desugar")
 might not even be necessary.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12950#comment:2>
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