Fix nasty bug in w/w for absence analysis

This dark corner was exposed by Trac #14285.  It involves the
interaction between absence analysis and INLINABLE pragmas.

There is a full explanation in Note [aBSENT_ERROR_ID] in MkCore,
which you can read there.  The changes in this patch are

* Make exprIsHNF return True for absentError, treating
  absentError like an honorary data constructor.

* Make absentError /not/ be diverging, unlike other error Ids.

This is all a bit horrible.

* While doing this I found that exprOkForSpeculation didn't
  have a case for value lambdas so I added one.  It's not
  really called on lifted types much, but it seems like the
  right thing

4 files changed, 48 insertions, 0 deletions

diff --git a/testsuite/tests/stranal/should_run/T14285.hs b/testsuite/tests/stranal/should_run/T14285.hs
new file mode 100644
index 0000000000..29da51e734
--- /dev/null
+++ b/testsuite/tests/stranal/should_run/T14285.hs
@@ -0,0 +1,9 @@
+module Main where
+
+import T14285a
+import Prelude hiding (null)
+
+main :: IO ()
+main = do
+  let args = "hw"
+  print $ null $ pre_images 'a' (Rel (fromList [('a',sfromList args)]) (fromList [('b',sfromList args)]))
diff --git a/testsuite/tests/stranal/should_run/T14285.stdout b/testsuite/tests/stranal/should_run/T14285.stdout
new file mode 100644
index 0000000000..0ca95142bb
--- /dev/null
+++ b/testsuite/tests/stranal/should_run/T14285.stdout
@@ -0,0 +1 @@
+True
diff --git a/testsuite/tests/stranal/should_run/T14285a.hs b/testsuite/tests/stranal/should_run/T14285a.hs
new file mode 100644
index 0000000000..8ee9b38df6
--- /dev/null
+++ b/testsuite/tests/stranal/should_run/T14285a.hs
@@ -0,0 +1,37 @@
+module T14285a where
+
+import qualified Data.Foldable as F
+import qualified Data.IntMap as IM
+import qualified Data.IntSet as IS
+import Prelude hiding (null)
+
+data Set k = Set IS.IntSet
+
+empty = Set IS.empty
+
+
+null (Set a) = IS.null a
+
+sfromList :: (Enum a, Foldable c) => c a -> Set a
+sfromList xs = Set $ IS.fromList $ Prelude.map fromEnum $ F.toList xs
+
+{-# inlineable fromList #-}
+fromList :: Enum k => [(k,v)] -> Map k v
+fromList kvs =
+  Map $ IM.fromList $ Prelude.map (\(k,v) -> (fromEnum k, v)) kvs
+
+
+newtype Map k v = Map { unMap :: (IM.IntMap v) } deriving (Eq, Ord)
+
+{-# inlineable findWithDefault #-}
+findWithDefault d k (Map m) = IM.findWithDefault d (fromEnum k) m
+
+data Rel a b = Rel !(Map a (Set b)) !(Map b (Set a))
+
+{-# INLINEABLE images #-}
+images x (Rel f b) = findWithDefault empty x f
+{-# INLINEABLE pre_images #-}
+pre_images x rel = images x $ mirrorRel rel
+{-# INLINEABLE mirrorRel #-}
+mirrorRel :: Rel a b -> Rel b a
+mirrorRel (Rel f g) = Rel g f
diff --git a/testsuite/tests/stranal/should_run/all.T b/testsuite/tests/stranal/should_run/all.T
index dada8176cb..a9cc3d7071 100644
--- a/testsuite/tests/stranal/should_run/all.T
+++ b/testsuite/tests/stranal/should_run/all.T
@@ -16,3 +16,4 @@ test('T12368', exit_code(1), compile_and_run, [''])
 test('T12368a', exit_code(1), compile_and_run, [''])
 test('T13380', [expect_broken(13380), exit_code(1)], compile_and_run, [''])
 test('T14290', normal, compile_and_run, [''])
+test('T14285', normal, multimod_compile_and_run, ['T14285', ''])