[GHC] #14951: SpecContsr needs two runs when one should suffice

[GHC]

#14951: SpecContsr needs two runs when one should suffice
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  SpecConstr
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14844            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by sgraf):

 Indeed we can only add `t ↦ ScrutOcc` if we know that a matching
 specialisation will apply. Consider what would happen if we regarded
 `(n-1)` as a constructor form (it isn't , of course, but imagine inductive
 nats), too:

 Then we would have `[ScrutOcc [UnkOcc], ScrutOcc [UnkOcc, UnkOcc]` for
 `l`s RHS and would have a more specific specialisation for `l (n-1)
 (x,y)`. If we bubble out a usage of `t ↦ ScrutOcc` within `foo` from the
 `l s' t` call, this will attempt to do a specialisation of `foo` when it
 hits a call site like in `bar1`. But now there's no matching
 specialisation of `l` anymore: The one which was a candidate before also
 assumes `n-1` as an argument. This can potentially make things much worse.
 So the whole signature thing seems like not such a good idea after all.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14951#comment:8>
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