Test for GADT escape (I forget which Trac bug this is for, if any)

testsuite/tests/ghc-regress/gadt/all.T

@@ -67,6 +67,7 @@ test('set', normal, compile, [''])
 
 test('scoped', expect_broken(1823), compile, [''])
 
+test('gadt-escape1', normal, compile_fail, [''])
 
 # New ones from Dimitrios
 

testsuite/tests/ghc-regress/gadt/gadt-escape1.hs

+{-# OPTIONS -XGADTs -XExistentialQuantification #-}
+
+module Escape where
+
+data ExpGADT t where
+  ExpInt :: Int -> ExpGADT Int
+
+data Hidden = forall t . Hidden (ExpGADT t) (ExpGADT t)
+
+hval = Hidden (ExpInt 0) (ExpInt 1)
+
+-- With the type sig this is ok, but without it should
+-- be rejected becuase the result type is wobbly
+--    weird1 :: ExpGADT Int
+
+weird1 = case (hval :: Hidden) of Hidden (ExpInt _) a -> a
+
+weird2 :: ExpGADT Int
+weird2 = case (hval :: Hidden) of Hidden (ExpInt _) a -> a

testsuite/tests/ghc-regress/gadt/gadt-escape1.stderr

+
+gadt-escape1.hs:16:52:
+    GADT pattern match with non-rigid result type `t'
+      Solution: add a type signature
+    In a case alternative: Hidden (ExpInt _) a -> a
+    In the expression:
+        case (hval :: Hidden) of Hidden (ExpInt _) a -> a
+    In the definition of `weird1':
+        weird1 = case (hval :: Hidden) of Hidden (ExpInt _) a -> a