Add test for Trac #3440

testsuite/tests/ghc-regress/indexed-types/should_fail/T3440.hs

+{-# LANGUAGE TypeFamilies, GADTs #-}
+
+module T3440 where
+
+type family Fam a :: *
+
+data GADT :: * -> * where
+   GADT :: a -> Fam a -> GADT (Fam a)
+
+unwrap :: GADT (Fam a) -> (a, Fam a)
+unwrap (GADT x y) = (x, y)

testsuite/tests/ghc-regress/indexed-types/should_fail/T3440.stderr

+
+T3440.hs:11:22:
+    Could not deduce (a1 ~ a) from the context (Fam a ~ Fam a1)
+      `a1' is a rigid type variable bound by
+           a pattern with constructor
+           GADT :: forall a. a -> Fam a -> GADT (Fam a)
+             at T3440.hs:11:9
+      `a' is a rigid type variable bound by
+          the type signature for `unwrap' at T3440.hs:10:21
+    In the expression: x
+    In the expression: (x, y)
+    In an equation for `unwrap': unwrap (GADT x y) = (x, y)

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

@@ -62,4 +62,4 @@ test('T3330c', normal, compile_fail, [''])
 test('T4179', normal, compile_fail, [''])
 test('T4254', normal, compile_fail, [''])
 test('T2239', normal, compile_fail, [''])
-
+test('T3440', normal, compile_fail, [''])