WorkWrap: Remove mkWWargs (#19874)

`mkWWargs`'s job was pushing casts inwards and doing eta expansion to match
the arity with the number of argument demands we w/w for.

Nowadays, we use the Simplifier to eta expand to arity. In fact, in recent years
we have even seen the eta expansion done by w/w as harmful, see Note [Don't eta
expand in w/w]. If a function hasn't enough manifest lambdas, don't w/w it!

What purpose does `mkWWargs` serve in this world? Not a great one, it turns out!
I could remove it by pulling some important bits,
notably Note [Freshen WW arguments] and Note [Join points and beta-redexes].
Result: We reuse the freshened binder names of the wrapper in the
worker where possible (see testuite changes), much nicer!

In order to avoid scoping errors due to lambda-bound unfoldings in worker
arguments, we zap those unfoldings now. In doing so, we fix #19766.

Fixes #19874.

3 files changed, 24 insertions, 4 deletions

diff --git a/testsuite/tests/stranal/should_compile/T16029.stdout b/testsuite/tests/stranal/should_compile/T16029.stdout
index 2d1f2106f3..76bf2617fb 100644
--- a/testsuite/tests/stranal/should_compile/T16029.stdout
+++ b/testsuite/tests/stranal/should_compile/T16029.stdout
@@ -4,8 +4,8 @@
   :: GHC.Prim.Int# -> GHC.Prim.Int#
   = \ (ww :: GHC.Prim.Int#) ->
 g2 [InlPrag=[2]] :: T -> Int -> Int
-         Tmpl= \ (w [Occ=Once1!] :: T) (w1 [Occ=Once1!] :: Int) ->
-  = \ (w :: T) (w1 :: Int) ->
+         Tmpl= \ (ds [Occ=Once1!] :: T) (ds1 [Occ=Once1!] :: Int) ->
+  = \ (ds :: T) (ds1 :: Int) ->
 g1 [InlPrag=[2]] :: S -> Int -> Int
-         Tmpl= \ (w [Occ=Once1!] :: S) (w1 [Occ=Once1!] :: Int) ->
-  = \ (w :: S) (w1 :: Int) ->
+         Tmpl= \ (ds [Occ=Once1!] :: S) (ds1 [Occ=Once1!] :: Int) ->
+  = \ (ds :: S) (ds1 :: Int) ->
diff --git a/testsuite/tests/stranal/should_compile/T19766.hs b/testsuite/tests/stranal/should_compile/T19766.hs
new file mode 100644
index 0000000000..1062c57cc1
--- /dev/null
+++ b/testsuite/tests/stranal/should_compile/T19766.hs
@@ -0,0 +1,19 @@
+{-# OPTIONS_GHC -O #-}
+
+module T19766 where
+
+data T a b = T !a !b
+
+data HasT = A (T Int Int) | B (T Int Int)
+
+getT :: HasT -> T Int Int
+getT (A t) = t
+getT (B t) = t
+
+f :: HasT -> [Int]
+f ht = case getT ht of t@(T _ _) -> reverse $ reverse $ reverse $ reverse $ reverse $ reverse $ lookupGRE t 15 [1,2,3,4]
+{-# NOINLINE f #-}
+
+lookupGRE :: T Int a -> Int -> [Int] -> [Int]
+lookupGRE ~(T n _) !k xs = [ x | x <- xs, x+k == n ]
+{-# NOINLINE lookupGRE #-}
diff --git a/testsuite/tests/stranal/should_compile/all.T b/testsuite/tests/stranal/should_compile/all.T
index f5ebbf289a..9a210ea165 100644
--- a/testsuite/tests/stranal/should_compile/all.T
+++ b/testsuite/tests/stranal/should_compile/all.T
@@ -66,6 +66,7 @@ test('T18894b',  [ grep_errmsg(r'Arity=2') ], compile, ['-ddump-stranal -dsuppre
 test('T18982',  [ grep_errmsg(r'\$w. .*Int#$') ], compile, ['-dppr-cols=1000 -ddump-simpl -dsuppress-idinfo -dsuppress-uniques'])
 
 test('T19180', normal, compile, [''])
+test('T19766', [ grep_errmsg(r'absentError') ], compile, ['-ddump-worker-wrapper'])
 test('T19849', normal, compile, [''])
 test('T19882a', normal, compile, [''])
 test('T19882b', normal, compile, [''])