This was a nasty, long-standing bug exposed in Trac #10538.
Symptoms were that we had an empty case
   case (x :: Either a) of {}
Core Lint correctly picked this bogus code up.

Here is what happened

* In SimplUtils.prepareAlts, we call
        filterAlts
  then
        combineIdenticalAlts

* We had    case x of { Left _ -> e1; Right _ -> e1 }

* filterAlts did nothing, but correctly retuned imposs_deflt_cons
  saying that 'x' cannot be {Left, Right} in the DEFAULT branch,
  if any (there isn't one.)

* combineIdentialAlts correctly combines the identical alts, to give
     case x of { DEFAULT -> e1 }

* BUT combineIdenticalAlts did no adjust imposs_deft_cons

* Result: when compiling e1 we did so in the belief that 'x'
  could not be {Left,Right}.  Disaster.

Easily fixed.

(It is hard to trigger; I can't construct a simple test case.)