GHC Handbook

Note [SpecConstr and strict fields]

GHC/Core/Opt/SpecConstr.hs:633 compiler 1 ticket 

We treat strict fields in SpecConstr the same way we do in W/W.
That is we make the specialized function strict in arguments
representing strict fields. See Note [Call-by-value for worker args]
for why we do this.

(SCF1) The arg_id might be an /imported/ Id like M.foo_acf (see #24944).
  We don't want to make
     case M.foo_acf of M.foo_acf { DEFAULT -> blah }
  because the binder of a case-expression should never be imported.  Rather,
  we must localise it thus:
     case M.foo_acf of foo_acf { DEFAULT -> blah }
  We keep the same unique, so in the next round of simplification we'll replace
  any M.foo_acf's in `blah` by `foo_acf`.

  c.f. Note [Localise pattern binders] in GHC.HsToCore.Utils.

References 2

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