Add test for Trac #4246

testsuite/tests/ghc-regress/indexed-types/should_fail/T4246.hs

+{-# LANGUAGE TypeFamilies, FlexibleInstances, OverlappingInstances #-}
+module T4246 where
+
+class Stupid a where
+   type F a
+
+instance Stupid a where
+   type F a = a
+
+instance Stupid Int where
+   type F Int = Bool
+
+type family G a :: *
+type instance G Int = Int
+type instance G Int = Bool

testsuite/tests/ghc-regress/indexed-types/should_fail/T4246.stderr

+
+T4246.hs:8:9:
+    Conflicting family instance declarations:
+      type instance F a -- Defined at T4246.hs:8:9
+      type instance F Int -- Defined at T4246.hs:11:9
+
+T4246.hs:14:15:
+    Conflicting family instance declarations:
+      type instance G Int -- Defined at T4246.hs:14:15
+      type instance G Int -- Defined at T4246.hs:15:15

testsuite/tests/ghc-regress/indexed-types/should_fail/all.T

@@ -53,3 +53,4 @@ test('NoMatchErr', normal, compile_fail, [''])
 test('T2677', normal, compile_fail, [''])
 test('T4099', normal, compile_fail, [''])
 test('T4272', normal, compile_fail, [''])
+test('T4246', normal, compile_fail, [''])