Add a test for #12600

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #12600

Differential Revision: https://phabricator.haskell.org/D3580

4 files changed, 39 insertions, 0 deletions

diff --git a/testsuite/tests/simplCore/should_compile/Makefile b/testsuite/tests/simplCore/should_compile/Makefile
index a01edb220d..f56a851597 100644
--- a/testsuite/tests/simplCore/should_compile/Makefile
+++ b/testsuite/tests/simplCore/should_compile/Makefile
@@ -225,3 +225,8 @@ T13340:
 T11272:
 	'$(TEST_HC)' $(TEST_HC_OPTS) -c -O T11272a.hs
 	'$(TEST_HC)' $(TEST_HC_OPTS) -c -O -ddump-prep T11272.hs | { ! grep Ord ;}
+
+# We expect to see a $wfoo worker that doesn't take any dictionaries.
+.PHONY: T12600
+T12600:
+	'$(TEST_HC)' $(TEST_HC_OPTS) -c -O -ddump-prep -dsuppress-all -dsuppress-uniques -dno-suppress-type-signatures -dppr-cols=200 T12600.hs | grep "wfoo" | head -n 1
diff --git a/testsuite/tests/simplCore/should_compile/T12600.hs b/testsuite/tests/simplCore/should_compile/T12600.hs
new file mode 100644
index 0000000000..d08d923ac6
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T12600.hs
@@ -0,0 +1,29 @@
+module T12600 where
+
+-- We don't want to see any dictionary-passing in foo. Everything
+-- should be inlined or specialized away.
+
+class Eq1 f where
+  eq1 :: Eq a => f a -> f a -> Bool
+
+data F a = F !a !a
+data G f a = G !(f a) !(f a)
+
+instance Eq1 F where
+  eq1 = \(F a b) (F c d) ->
+    -- In order to reproduce the problem, the body of this function needs to be
+    -- large enough to prevent GHC from voluntarily inlining it.
+    larger $ larger $ larger $ larger $ larger $ larger $
+      a == c && b == d
+  {-# INLINE eq1 #-}
+
+larger :: a -> a
+larger = id
+{-# NOINLINE larger #-}
+
+instance (Eq1 f) => Eq1 (G f) where
+  eq1 = \(G a b) (G c d) -> eq1 a c && eq1 b d
+  {-# INLINE eq1 #-}
+
+foo :: G F Int -> G F Int -> Bool
+foo a b = eq1 a b
diff --git a/testsuite/tests/simplCore/should_compile/T12600.stdout b/testsuite/tests/simplCore/should_compile/T12600.stdout
new file mode 100644
index 0000000000..94118740f7
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T12600.stdout
@@ -0,0 +1 @@
+$wfoo :: Int# -> Int# -> Int# -> Int# -> Int# -> Int# -> Int# -> Int# -> Bool
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 1af5cbed25..b8a0c66d7f 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -264,3 +264,7 @@ test('T11272',
      normal,
      run_command,
      ['$MAKE -s --no-print-directory T11272'])
+test('T12600',
+     normal,
+     run_command,
+     ['$MAKE -s --no-print-directory T12600'])