Arity: Rework `ArityType` to fix monotonicity (#18870)

As we found out in #18870, `andArityType` is not monotone, with
potentially severe consequences for termination of fixed-point
iteration. That showed in an abundance of "Exciting arity" DEBUG
messages that are emitted whenever we do more than one step in
fixed-point iteration.

The solution necessitates also recording `OneShotInfo` info for
`ABot` arity type. Thus we get the following definition for `ArityType`:

```
data ArityType = AT [OneShotInfo] Divergence
```

The majority of changes in this patch are the result of refactoring use
sites of `ArityType` to match the new definition.

The regression test `T18870` asserts that we indeed don't emit any DEBUG
output anymore for a function where we previously would have.
Similarly, there's a regression test `T18937` for #18937, which we
expect to be broken for now.

Fixes #18870.

3 files changed, 22 insertions, 0 deletions

diff --git a/testsuite/tests/arityanal/should_compile/T18870.hs b/testsuite/tests/arityanal/should_compile/T18870.hs
new file mode 100644
index 0000000000..b94fd13ffb
--- /dev/null
+++ b/testsuite/tests/arityanal/should_compile/T18870.hs
@@ -0,0 +1,12 @@
+{-# OPTIONS_GHC -O2 -fforce-recomp #-}
+
+module T18870 where
+
+import GHC.Exts
+
+-- This function should not lead to an "Exciting arity" DEBUG message.
+-- It should only do one round of fixed-point iteration to conclude that it has
+-- arity 2.
+f :: [a] -> a -> a
+f []     = id
+f (x:xs) = oneShot (\_ -> f xs x)
diff --git a/testsuite/tests/arityanal/should_compile/T18937.hs b/testsuite/tests/arityanal/should_compile/T18937.hs
new file mode 100644
index 0000000000..c7db70af02
--- /dev/null
+++ b/testsuite/tests/arityanal/should_compile/T18937.hs
@@ -0,0 +1,8 @@
+{-# OPTIONS_GHC -O2 -fforce-recomp #-}
+
+module T18937 where
+
+f :: [Int] -> Int -> Int
+f []     = id
+f (x:xs) = let y = sum [0..x]
+           in \z -> f xs (y + z)
diff --git a/testsuite/tests/arityanal/should_compile/all.T b/testsuite/tests/arityanal/should_compile/all.T
index 3413a3270c..60059b8e9c 100644
--- a/testsuite/tests/arityanal/should_compile/all.T
+++ b/testsuite/tests/arityanal/should_compile/all.T
@@ -19,3 +19,5 @@ test('Arity16', [ only_ways(['optasm']), grep_errmsg('Arity=') ], compile, ['-dn
 
 # Regression tests
 test('T18793', [ only_ways(['optasm']), grep_errmsg('Arity=') ], compile, ['-dno-typeable-binds -ddump-simpl -dppr-cols=99999 -dsuppress-uniques'])
+test('T18870', [ only_ways(['optasm']) ], compile, ['-ddebug-output'])
+test('T18937', [ only_ways(['optasm']), when(compiler_debugged(), expect_broken(18937)) ], compile, ['-ddebug-output'])