Fix #16518 with some more kind-splitting smarts

This patch corrects two simple oversights that led to #16518:

1. `HsUtils.typeToLHsType` was taking visibility into account in the
   `TyConApp` case, but not the `AppTy` case. I've factored out the
   visibility-related logic into its own `go_app` function and now
   invoke `go_app` from both the `TyConApp` and `AppTy` cases.
2. `Type.fun_kind_arg_flags` did not properly split kinds with
   nested `forall`s, such as
   `(forall k. k -> Type) -> (forall k. k -> Type)`. This was simply
   because `fun_kind_arg_flags`'s `FunTy` case always bailed out and
   assumed all subsequent arguments were `Required`, which clearly
   isn't the case for nested `forall`s. I tweaked the `FunTy` case
   to recur on the result kind.

2 files changed, 37 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T16518.hs b/testsuite/tests/deriving/should_compile/T16518.hs
new file mode 100644
index 0000000000..49efe34673
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T16518.hs
@@ -0,0 +1,36 @@
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE TypeOperators #-}
+module T16518 where
+
+import Data.Coerce
+import Data.Kind
+import Data.Type.Equality
+
+-----
+
+class HTestEquality1 (f :: forall k. k -> Type) where
+  hTestEquality1 :: forall k1 k2 (a :: k1) (b :: k2).
+                    f a -> f b -> Maybe (a :~~: b)
+newtype T1 :: (forall k. k -> Type) -> (forall k. k -> Type) where
+  MkT1 :: forall (f :: forall k. k -> Type) k (a :: k). f a -> T1 f a
+
+deriving instance forall (f :: forall k. k -> Type).
+                  HTestEquality1 f => HTestEquality1 (T1 f)
+
+-----
+
+class HTestEquality2 (f :: forall k -> k -> Type) where
+  hTestEquality2 :: forall k1 k2 (a :: k1) (b :: k2).
+                    f k1 a -> f k2 b -> Maybe (a :~~: b)
+newtype T2 :: (forall k -> k -> Type) -> (forall k -> k -> Type) where
+  MkT2 :: forall (f :: forall k -> k -> Type) k (a :: k). f k a -> T2 f k a
+
+deriving instance forall (f :: forall k -> k -> Type).
+                  HTestEquality2 f => HTestEquality2 (T2 f)
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 23f152e195..a5f666c062 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -115,3 +115,4 @@ test('T15290d', normal, compile, [''])
 test('T15398', normal, compile, [''])
 test('T15637', normal, compile, [''])
 test('T16179', normal, compile, [''])
+test('T16518', normal, compile, [''])