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exprIsDeadEnd :: CoreExpr -> Bool
-- See Note [Bottoming expressions]
exprIsDeadEnd e
  | isEmptyTy (exprType e)
  = True
  | otherwise
  = go 0 e
  where
    go n (Var v)                 = isDeadEndId v &&  n >= idArity v
    go n (App e a) | isTypeArg a = go n e
                   | otherwise   = go (n+1) e

We have

failIO :: forall a. String -> IO a
[Arity = 0 -- unknown arity
, ...]
failIO = failIO1

We call

exprIsDead (App failIO msg)
=> AppCase
go 1 failIO 
=> Var case
isDeadEndId failIO &&  1 >= idArity failIO
=>
True &&  n >= 1 >= 0
=>
True

The problem is that we trust idArity to return a known arity. But it can also return zero if the arity is unknown.

That means for a potentially bottoming function we always get True for exprIsDeadEnd if it has unknown Arity.