Case-of-case not eliminated when it could be

Small example:

foo :: Int -> Int
foo n = (if n < 5 then error "Too small" else n+2) `seq` n+5

GHC produces this:

foo =
  \ (w_slQ :: Int) ->
    case w_slQ of _ { I# ww_slT ->
    case case <# ww_slT 5 of _ {
           False -> case +# ww_slT 2 of sat_sm1 { __DEFAULT -> I# sat_sm1 };
           True -> foo1
         }
    of _ { I# _ ->
    case +# ww_slT 5 of sat_sm4 { __DEFAULT -> I# sat_sm4 }
    }
    }

It ought to produce something like this instead:

foo n = case n of I# n# ->
        case n# <# 5 of
          True -> foo1
          False -> I# (n# +# 5)

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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