Test for Trac #1662

testsuite/tests/ghc-regress/arrows/should_fail/all.T

@@ -3,3 +3,4 @@ setTestOpts(only_compiler_types(['ghc']))
 test('arrowfail001', normal, compile_fail, [''])
 test('arrowfail002', normal, compile_fail, [''])
 test('arrowfail003', normal, compile_fail, [''])
+test('arrowfail004', normal, compile_fail, [''])

testsuite/tests/ghc-regress/arrows/should_fail/arrowfail004.hs

+{-# LANGUAGE Arrows, ExistentialQuantification #-}
+
+-- Trac #1662
+
+module ShouldFail where
+
+import Control.Arrow
+
+data T = forall a. T a
+
+panic :: (Arrow arrow) => arrow T T
+panic = proc (T x) -> do returnA -< T x
\ No newline at end of file

testsuite/tests/ghc-regress/arrows/should_fail/arrowfail004.stderr

+
+arrowfail004.hs:12:14:
+    Illegal constructor `T' in a 'proc' pattern
+      Proc patterns cannot use existentials or GADTs
+    In the pattern: T x
+    In the expression: proc (T x) -> do returnA -< T x
+    In the definition of `panic':
+        panic = proc (T x) -> do returnA -< T x