From ff21795a0b9253e811a45626d5686e981ed07f82 Mon Sep 17 00:00:00 2001
From: Simon Peyton Jones <simonpj@microsoft.com>
Date: Fri, 22 Jan 2016 16:34:18 +0000
Subject: Special-case implicit params in superclass expansion

This issue came up in Trac #11480, and is documented in
Note [When superclasses help] in TcRnTypes.

We were getting a spurious warning
  T11480.hs:1:1: warning:
     solveWanteds: too many iterations (limit = 4)

The fix is easy.  A bit of refactoring along the way.

The original bug report in Trac #11480 appears to work
fine in HEAD and the 8.0 branch but I added a regression
test in this commit as well.
---
 testsuite/tests/polykinds/T11480a.hs | 26 ++++++++++++++++++++++++++
 testsuite/tests/polykinds/all.T      |  1 +
 2 files changed, 27 insertions(+)
 create mode 100644 testsuite/tests/polykinds/T11480a.hs

(limited to 'testsuite/tests/polykinds')

diff --git a/testsuite/tests/polykinds/T11480a.hs b/testsuite/tests/polykinds/T11480a.hs
new file mode 100644
index 0000000000..3d17168082
--- /dev/null
+++ b/testsuite/tests/polykinds/T11480a.hs
@@ -0,0 +1,26 @@
+{-# language KindSignatures, PolyKinds, TypeFamilies,
+  NoImplicitPrelude, FlexibleContexts,
+  MultiParamTypeClasses, GADTs,
+  ConstraintKinds, FlexibleInstances,
+  FunctionalDependencies, UndecidableSuperClasses #-}
+
+module T11480a where
+
+import GHC.Types (Constraint)
+import qualified Prelude
+
+data Nat (c :: i -> i -> *) (d :: j -> j -> *) (f :: i -> j) (g :: i -> j)
+
+class Functor p (Nat p (->)) p => Category (p :: i -> i -> *)
+
+class (Category dom, Category cod)
+   => Functor (dom :: i -> i -> *) (cod :: j -> j -> *) (f :: i -> j)
+    | f -> dom cod
+
+instance (Category c, Category d) => Category (Nat c d)
+instance (Category c, Category d) => Functor (Nat c d) (Nat (Nat c d) (->)) (Nat c d)
+instance (Category c, Category d) => Functor (Nat c d) (->) (Nat c d f)
+
+instance Category (->)
+instance Functor (->) (->) ((->) e)
+instance Functor (->) (Nat (->) (->)) (->)
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index f1f25cecc4..69c5ba0790 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -134,3 +134,4 @@ test('T11278', normal, compile, [''])
 test('T11255', normal, compile, [''])
 test('T11459', normal, compile_fail, [''])
 test('T11466', normal, compile_fail, [''])
+test('T11480a', normal, compile, [''])
-- 
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