Test #8428

testsuite/tests/typecheck/should_fail/T8428.hs

+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ImpredicativeTypes #-}
+
+module T8428 where
+
+import Control.Monad.ST
+
+data IdentityT m a = IdentityT { runIdentityT :: m a }
+
+runIdST :: IdentityT (forall s. ST s) a -> a
+runIdST = runST . runIdentityT
+-- Test formatting in the error message.
+-- In fact this should be rejected as a kind error (#8388)

testsuite/tests/typecheck/should_fail/T8428.stderr

+
+T8428.hs:11:19:
+    Couldn't match type ‛(forall s. ST s) a’ with ‛forall s. ST s a’
+    Expected type: IdentityT (forall s. ST s) a -> forall s. ST s a
+      Actual type: IdentityT (forall s. ST s) a -> (forall s. ST s) a
+    Relevant bindings include
+      runIdST :: IdentityT (forall s. ST s) a -> a
+        (bound at T8428.hs:11:1)
+    In the second argument of ‛(.)’, namely ‛runIdentityT’
+    In the expression: runST . runIdentityT

testsuite/tests/typecheck/should_fail/all.T

@@ -321,4 +321,4 @@ test('TcCoercibleFailSafe', when(compiler_lt('ghc', '7.7'), skip), compile_fail,
 test('TcCoercibleFail2', when(compiler_lt('ghc', '7.7'), skip), compile_fail, [''])
 test('T8306', normal, compile_fail, [''])
 test('T8392a', normal, compile_fail, [''])
-
+test('T8428', normal, compile_fail, [''])