Note [aBSENT_ERROR_ID]

We use aBSENT_ERROR_ID to build absent fillers for lifted types in workers. E.g.

   f x = (case x of (a,b) -> b) + 1::Int

The demand analyser figures out that only the second component of x is
used, and does a w/w split thus

   f x = case x of (a,b) -> $wf b

   $wf b = let a = absentError "blah"
               x = (a,b)
           in <the original RHS of f>

After some simplification, the (absentError "blah") thunk normally goes away.
See also Note [Absent fillers] in GHC.Core.Opt.WorkWrap.Utils.

Historical Note

We used to have exprIsHNF respond True to absentError and *not* mark it as diverging.
Here's the reason for the former. It doesn't apply anymore because we no longer say
that `a` is absent (A). Instead it gets (head strict) demand 1A and we won't
emit the absent error:

#14285 had, roughly

   data T a = MkT a !a
   {-# INLINABLE f #