Improve CPR behavior for strict constructors

When working on Trac #10482 I noticed that we could give constructor
arguments the CPR property if they are use strictly.

This is documented carefully in
    Note [CPR in a product case alternative]
and also
    Note [Initial CPR for strict binders]

There are a bunch of intersting examples in
    Note [CPR examples]
which I have added to the test suite as T10482a.

I also added a test for #10482 itself.

1 files changed, 14 insertions, 0 deletions

diff --git a/testsuite/tests/stranal/should_compile/T10482.hs b/testsuite/tests/stranal/should_compile/T10482.hs
new file mode 100644
index 0000000000..ef7c29c4be
--- /dev/null
+++ b/testsuite/tests/stranal/should_compile/T10482.hs
@@ -0,0 +1,14 @@
+{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE TypeFamilies #-}
+module T10482 where
+
+data family Foo a
+data instance Foo (a, b) = FooPair !(Foo a) !(Foo b)
+newtype instance Foo Int = Foo Int
+
+foo :: Foo ((Int, Int), Int) -> Int -> Int
+foo !f k =
+  if k == 0 then 0
+  else if even k then foo f (k-1)
+  else case f of
+    FooPair (FooPair (Foo n) _) _ -> n