[GHC] #15226: GHC doesn't know that seq# produces something in WHNF

[GHC]

#15226: GHC doesn't know that seq# produces something in WHNF
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  merge
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4796
       Wiki Page:                    |
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Comment (by dfeuer):

 The key `OccAnal` functions involved seem to be `mkAltEnv` (not sure what
 this does, but it seems to look for `Var`s when I want to look also for
 applications of `seq#`) and `wrapAltRHS` (which seems to actually install
 the `Let` when appropriate), but I don't see how it all fits together.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15226#comment:10>
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