Fix the superclass-cycle detection code (Trac #9739)

We were falling into an infinite loop when doing the ambiguity
check on a class method, even though we had previously detected
a superclass cycle.  There was code to deal with this, but it
wasn't right.

2 files changed, 11 insertions, 8 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T9739.hs b/testsuite/tests/typecheck/should_fail/T9739.hs
index 4b7869d315..18df797100 100644
--- a/testsuite/tests/typecheck/should_fail/T9739.hs
+++ b/testsuite/tests/typecheck/should_fail/T9739.hs
@@ -1,6 +1,9 @@
+{-# LANGUAGE MultiParamTypeClasses #-}
 module T9739 where
 
-class Class2 a => Class1 a where
-  class3 :: (Class2 a) => b
+class Class3 a => Class1 a where
 
-class (Class1 a) => Class2 a where
+class Class2 t a where
+  class2 :: (Class3 t) => a -> m
+
+class (Class1 t, Class2 t t) => Class3 t where
diff --git a/testsuite/tests/typecheck/should_fail/T9739.stderr b/testsuite/tests/typecheck/should_fail/T9739.stderr
index 95fcf6ad82..34e2f114f8 100644
--- a/testsuite/tests/typecheck/should_fail/T9739.stderr
+++ b/testsuite/tests/typecheck/should_fail/T9739.stderr
@@ -1,10 +1,10 @@
 
-T9739.hs:3:1:
+T9739.hs:4:1:
     Cycle in class declaration (via superclasses):
-      Class1 -> Class2 -> Class1
+      Class1 -> Class3 -> Class1
     In the class declaration for ‘Class1’
 
-T9739.hs:6:1:
+T9739.hs:9:1:
     Cycle in class declaration (via superclasses):
-      Class2 -> Class1 -> Class2
-    In the class declaration for ‘Class2’
+      Class3 -> Class1 -> Class3
+    In the class declaration for ‘Class3’