Note [Lone variables] leads to missing a case-of-case opportunity

After the simplifier, my program ends up in the following state.

foo = \f s -> let x = case s of { T1 a b -> a :> b :> Nil }
              in fmap (\v -> case x of 
                              { _v1 :> vs -> T1 v (case vs of {v2 :> _ -> v2}) }) 
                      (f (case x of {v1 :> _ -> v1}))

Now if I understand Note [Lone variables] correctly, x is NOT inlined into the call sites, no matter what I do as x is work-free. However, this is bad as if we were to inline x we get a case-of-case opportunity.

=> Inline
foo = \f s -> let x = case s of { T1 a b -> a :> b :> Nil }
              in fmap (\v -> case (case s of { T1 a b -> a :> b :> Nil }) of 
                              { _v1 :> vs -> T1 v (case vs of {v2 :> _ -> v2}) }) 
                      (f (case x of {v1 :> _ -> v1}))
=> case of case
foo = \f s -> let x = case s of { T1 a b -> a :> b :> Nil }
              in fmap (\v -> case s of { T1 a b -> case (a :> b :> Nil) of 
                                                        { _v1 :> vs -> T1 v (case vs of {v2 :> _ -> v2}) }) 
                      (f (case x of {v1 :> _ -> v1}))
=> case of known constructor
foo = \f s -> let x = case s of { T1 a b -> a :> b :> Nil }
              in fmap (\v -> case s of { T1 a b -> T1 v b}) 
                      (f (case x of {v1 :> _ -> v1}))
=> Same for the other branch
foo = \f s -> fmap (\v -> case s of { T1 a b -> T1 v b}) 
                   (case s of T1 a b -> a)

Which no longer mentions the intermediate representation.

Trac metadata

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