[GHC] #14742: Unboxed sums can treat Word#s as Int#s

[GHC]

#14742: Unboxed sums can treat Word#s as Int#s
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        Reporter:  duog              |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  UnboxedSums
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by osa1):

 I think this is by design and we should relax the STG lint check if it's
 becoming a problem (or maybe we can add a special case for unboxed sums).

 The whole point of unboxed sums is to have a compact and unboxed layout.
 Compactness here means same memory slot (in registers or stack or heap
 locations) should be able to used for values of different types.
 `primRepSlot` is what decides what slots can a value be put into in an
 unboxed sum, and mapping larger number of prim reps to smaller number of
 slot types means we can share same slot for values of different types/prim
 reps.

 This seemingly ill-typed STG happens because we have to give unboxed sum
 data cons types, but we can't really say "anything that fits into a word
 slot" in our current type system, so we give it the type `Word`. This
 leads to seemingly ill-typed STG.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14742#comment:3>
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