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See this Note in GHC.Core.Utils

Note [Case expressions are work-free]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Are case-expressions work-free?  Consider
    let v = case x of (p,q) -> p
        go = \y -> ...case v of ...
Should we inline 'v' at its use site inside the loop?  At the moment
we do.  I experimented with saying that case are *not* work-free, but
that increased allocation slightly.  It's a fairly small effect, and at
the moment we go for the slightly more aggressive version which treats
(case x of ....) as work-free if the alternatives are.

Moreover it improves arities of overloaded functions where
there is only dictionary selection (no construction) involved

But when I was looking at the compiled code for mkTyConApp I found that code like

  let key = case tc of
              FunTyCon u _ _ _ -> u
              AlgTyCon u _ _ _ -> u
              etc
   in 
   ...key...key...key...

was inlining that key at all the call sites, and thereby duplicating the eval of tc and the case-branching involved. We should just do this once.

One solution: reverse the above Note. Another, less dramatic one: restrict it to cases where there is just one alternative.