STG CSE makes dead binders undead

Consider the following example:

go :: (a -> b) -> Either String a -> Either String b
go f (Right a) = Right (f a)
go _ (Left e)  = Left e

GHC with -O2 converts it into the following STG:

TestUndead.go
  :: forall a b.
     (a -> b)
     -> Data.Either.Either GHC.Base.String a
     -> Data.Either.Either GHC.Base.String b
[GblId,
 Arity=2,
 Caf=NoCafRefs,
 Str=<L,1*C1(U)><S,1*U>,
 Unf=OtherCon []] =
    \r [f_s17n ds_s17o]
        case ds_s17o of {
          Data.Either.Left e_s17q [Occ=Once] -> wild_s17p;
          Data.Either.Right a1_s17r [Occ=Once] ->
              let {
                sat_s17s [Occ=Once] :: b_aVN
                [LclId] =
                    \u [] f_s17n a1_s17r;
              } in  Data.Either.Right [sat_s17s];
        };

Notice that the dead binder wild_s17p is now alive (in the first alternative) but it isn't shown in case ds_s17o of { because the pretty-printer still assumes it is dead.

I think that in stgCseExpr .. (StgCase ...) (simplStg/StgCse.hs) we should check if the new binder is alive in the new alternatives, just like we do in coreToStgExpr (Case ...) (stgSyn/CoreToStg.hs), and use setIdOccInfo (ManyOccs NoTailCallInfo) on the new binder if necessary.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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