Opportunity to improve case-of-case

Consider this code

f g x = (case g x of True  -> not
                     False -> id) `seq` True

With GHC 8 (and -O) we get this Core

f =
  \ (@ t_a1rk) (g_aqD :: t -> Bool) (x_aqE :: t) ->
    case case g_aqD x_aqE of {
           False -> id @ Bool;
           True -> not
         }
    of
    { __DEFAULT ->
    GHC.Types.True
    }

This is obviously bad.

The reasoning is in the long, complicated Note [Single-alternative cases] in Simplify.hs.

Happily with join points the entire Note becomes unnecessary; we can delete the special case that this Note describes, and the Note itself. Result: simpler compiler, and more optimal code.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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