From 8426a1364ba450fe48fc41a95b2ba76c8d1bb7c8 Mon Sep 17 00:00:00 2001
From: Krzysztof Gogolewski <krzysztof.gogolewski@tweag.io>
Date: Mon, 24 Aug 2020 16:13:18 +0200
Subject: Add a test for #18585

---
 testsuite/tests/typecheck/should_compile/T18585.hs | 48 ++++++++++++++++++++++
 testsuite/tests/typecheck/should_compile/all.T     |  1 +
 2 files changed, 49 insertions(+)
 create mode 100644 testsuite/tests/typecheck/should_compile/T18585.hs

diff --git a/testsuite/tests/typecheck/should_compile/T18585.hs b/testsuite/tests/typecheck/should_compile/T18585.hs
new file mode 100644
index 0000000000..634675f5f5
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T18585.hs
@@ -0,0 +1,48 @@
+{-# Language DataKinds #-}
+{-# Language FlexibleContexts #-}
+{-# Language PolyKinds #-}
+{-# Language StandaloneKindSignatures #-}
+{-# Language TypeFamilyDependencies #-}
+{-# Language UndecidableInstances #-}
+{-# Language UndecidableSuperClasses #-}
+module T18585 (Functor(..)) where
+
+import Data.Kind (Type)
+import Prelude hiding (Functor(..))
+
+type Cat i = i -> i -> Type
+
+class
+  ( Op (Op k) ~ k
+  , Category (Op k)
+  ) => Category (k :: Cat i) where
+  type Op k :: i -> i -> Type
+  type Op k = Y k
+
+newtype Y k a b = Y (k b a)
+
+instance (Category k, Op k ~ Y k) => Category (Y k) where
+  type Op (Y k) = k
+
+instance Category (->)
+
+type SelfDom :: (i -> j) -> Cat i -> Cat i
+type family SelfDom (f :: i -> j) (k :: Cat i) :: Cat i where
+  SelfDom p p = Op p
+  SelfDom f p = p
+
+type family DefaultCat (i :: Type) = (res :: Cat i) | res -> i
+type instance DefaultCat Type = (->)
+
+class
+  ( Category (Dom f)
+  , Category (Cod f)
+  ) => Functor (f :: i -> j) where
+
+  type Dom f :: Cat i
+  type Dom (f :: i -> j) = SelfDom f (DefaultCat i)
+
+  type Cod f :: Cat j
+  type Cod (f :: i -> j) = DefaultCat j
+
+instance Functor IO
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index b2dc105b60..9e59160ccf 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -721,4 +721,5 @@ test('T18118', normal, multimod_compile, ['T18118', '-v0'])
 test('T18412', normal, compile, [''])
 test('T18470', normal, compile, [''])
 test('T18323', normal, compile, [''])
+test('T18585', normal, compile, [''])
 
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