Fix #14991.

It turns out that solveEqualities really does need to use simpl_top.
I thought that solveWanteds would be enough, and no existing test
case showed up the different. #14991 shows that we need simpl_top.
Easy enough to fix.

test case: dependent/should_compile/T14991

2 files changed, 35 insertions, 0 deletions

diff --git a/testsuite/tests/dependent/should_compile/T14991.hs b/testsuite/tests/dependent/should_compile/T14991.hs
new file mode 100644
index 0000000000..f435c37690
--- /dev/null
+++ b/testsuite/tests/dependent/should_compile/T14991.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE TypeInType #-}
+{-# LANGUAGE TypeOperators #-}
+{-# LANGUAGE UndecidableInstances #-}
+module T14991 where
+
+import Data.Kind
+
+type family Promote (k :: Type) :: Type
+type family PromoteX (a :: k) :: Promote k
+
+type family Demote (k :: Type) :: Type
+type family DemoteX (a :: k) :: Demote k
+
+-----
+-- Type
+-----
+
+type instance Demote Type = Type
+type instance Promote Type = Type
+
+type instance DemoteX (a :: Type) = Demote a
+type instance PromoteX (a :: Type) = Promote a
+
+-----
+-- Arrows
+-----
+
+data TyFun :: Type -> Type -> Type
+type a ~> b = TyFun a b -> Type
+infixr 0 ~>
+
+type instance Demote  (a ~> b) = DemoteX  a -> DemoteX  b
+type instance Promote (a -> b) = PromoteX a ~> PromoteX b
diff --git a/testsuite/tests/dependent/should_compile/all.T b/testsuite/tests/dependent/should_compile/all.T
index 070e1203f8..701e187b36 100644
--- a/testsuite/tests/dependent/should_compile/all.T
+++ b/testsuite/tests/dependent/should_compile/all.T
@@ -34,3 +34,4 @@ test('T14556', normal, compile, [''])
 test('T14720', normal, compile, [''])
 test('T14066a', normal, compile, [''])
 test('T14749', normal, compile, [''])
+test('T14991', normal, compile, [''])