GHC Handbook

Note [Preserve evaluatedness]

GHC/Core/Tidy.hs:411 compiler 

Consider
  data T = MkT !Bool
  ....(case v of MkT y ->
       let z# = case y of
                  True -> 1#
                  False -> 2#
       in ...)

The z# binding is ok because the RHS is ok-for-speculation,
but Lint will complain unless it can *see* that.  So we
preserve the evaluated-ness on 'y' in tidyBndr.

(Another alternative would be to tidy unboxed lets into cases,
but that seems more indirect and surprising.)

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