DeriveFoldable for data types with existential constraints (#10447)

Reviewers: dolio, shachaf, ekmett, austin, #core_libraries_committee,
simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1031

GHC Trac Issues: #10447

3 files changed, 51 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_run/T10447.hs b/testsuite/tests/deriving/should_run/T10447.hs
new file mode 100644
index 0000000000..e91ce98f64
--- /dev/null
+++ b/testsuite/tests/deriving/should_run/T10447.hs
@@ -0,0 +1,41 @@
+{-# LANGUAGE DeriveFoldable, GADTs, StandaloneDeriving #-}
+module Main where
+
+class (a ~ Int) => Foo a
+instance Foo Int
+
+data A a where
+  A1 :: Ord a            => a        -> A a
+  A2 ::                     Int      -> A Int
+  A3 :: b ~ Int          => b        -> A Int
+  A4 :: a ~ Int          => Int      -> A a
+  A5 :: a ~ Int          => a        -> A a
+  A6 :: (a ~ b, b ~ Int) => Int -> b -> A a
+  A7 :: Foo a            => Int -> a -> A a
+
+deriving instance Foldable A
+
+data HK f a where
+  HK1 :: f a -> HK f (f a)
+  HK2 :: f a -> HK f a
+
+deriving instance Foldable f => Foldable (HK f)
+
+one :: Int
+one = 1
+
+main :: IO ()
+main = do
+  mapM_ (print . foldr (+) one)
+    [ A1 one
+    , A2 one
+    , A3 one
+    , A4 one
+    , A5 one
+    , A6 one one
+    , A7 one one
+    ]
+  mapM_ (print . foldr mappend Nothing)
+    [ HK1 (Just "Hello")
+    , HK2 (Just (Just "World"))
+    ]
diff --git a/testsuite/tests/deriving/should_run/T10447.stdout b/testsuite/tests/deriving/should_run/T10447.stdout
new file mode 100644
index 0000000000..079b327601
--- /dev/null
+++ b/testsuite/tests/deriving/should_run/T10447.stdout
@@ -0,0 +1,9 @@
+2
+1
+1
+1
+2
+1
+2
+Nothing
+Just "World"
diff --git a/testsuite/tests/deriving/should_run/all.T b/testsuite/tests/deriving/should_run/all.T
index 1ccbdd77f8..d47e5c1312 100644
--- a/testsuite/tests/deriving/should_run/all.T
+++ b/testsuite/tests/deriving/should_run/all.T
@@ -39,3 +39,4 @@ test('T7931', normal, compile_and_run, [''])
 test('T9576', exit_code(1), compile_and_run, [''])
 test('T9830', extra_clean(['T9830a.hi', 'T9830a.o']), multimod_compile_and_run, ['T9830','-v0'])
 test('T10104', normal, compile_and_run, [''])
+test('T10447', normal, compile_and_run, [''])