+Note [Absent errors]
+~~~~~~~~~~~~~~~~~~~~
+We make a new binding for Ids that are marked absent, thus
+ let x = absentError "x :: Int"
+The idea is that this binding will never be used; but if it
+buggily is used we'll get a runtime error message.
+
+Coping with absence for *unlifted* types is important; see, for
+example, Trac #4306. For these we find a suitable literal,
+using Literal.absentLiteralOf. We don't have literals for
+every primitive type, so the function is partial.
+
+ [I did try the experiment of using an error thunk for unlifted
+ things too, relying on the simplifier to drop it as dead code,
+ by making absentError
+ (a) *not* be a bottoming Id,
+ (b) be "ok for speculation"
+ But that relies on the simplifier finding that it really
+ is dead code, which is fragile, and indeed failed when
+ profiling is on, which disables various optimisations. So
+ using a literal will do.]