Test Trac #9739

testsuite/tests/typecheck/should_fail/T9739.hs

+module T9739 where
+
+class Class2 a => Class1 a where
+  class3 :: (Class2 a) => b
+
+class (Class1 a) => Class2 a where

testsuite/tests/typecheck/should_fail/T9739.stderr

+
+T9739.hs:3:1:
+    Cycle in class declaration (via superclasses):
+      Class1 -> Class2 -> Class1
+    In the class declaration for ‘Class1’
+
+T9739.hs:6:1:
+    Cycle in class declaration (via superclasses):
+      Class2 -> Class1 -> Class2
+    In the class declaration for ‘Class2’

testsuite/tests/typecheck/should_fail/all.T

@@ -341,3 +341,4 @@ test('T9323', normal, compile_fail, [''])
 test('T9415', normal, compile_fail, [''])
 test('T9612', normal, compile_fail, [''])
 test('T9634', normal, compile_fail, [''])
+test('T9739', normal, compile_fail, [''])