Kind variables in RHS of an associated type instances should be bound on LHS

This patche fixes Trac #9574.

The previous Note [Renaming associated types] in RnTypes appears to me to be wrong;
it confused class and instance declarations.

I have:

 * Treated kind and type variables uniformly. Both must be bound on the LHS
   of an associated type instance.  Eg
       instance C ('KProxy :: KProxy o) where
          type F 'KProxy = NatTr (Proxy :: o -> *)
   is illegal because 'o' is not bound on the LHS of the instance.

 * Moved the Note to RnSource and fixed it up

This improves the error message from T7938. However it made the code in
T6118 incorrect. We had:
  instance SingE (a :: Maybe k) where
    type Demote a = Maybe (Demote (Any :: k))
and that is now rejected, rightly I think.

1 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T9574.hs b/testsuite/tests/polykinds/T9574.hs
new file mode 100644
index 0000000000..e806e2a6a2
--- /dev/null
+++ b/testsuite/tests/polykinds/T9574.hs
@@ -0,0 +1,18 @@
+{-# LANGUAGE PolyKinds, DataKinds, TypeFamilies, ScopedTypeVariables, GADTs, RankNTypes #-}
+module T9574 where
+
+data KProxy (t :: *) = KProxy
+data Proxy p
+
+class Funct f where
+    type Codomain f :: *
+
+instance Funct ('KProxy :: KProxy o) where
+    type Codomain 'KProxy = NatTr (Proxy :: o -> *)
+
+data NatTr (c :: o -> *) where
+    M :: (forall (a :: o). Proxy a) -> NatTr (c :: o -> *)
+
+p :: forall (c :: o -> *). NatTr c
+p = M t where
+    M t = undefined :: Codomain ('KProxy :: KProxy o)