Add test cases for #7503, #14451

At some point between 8.4 and 8.6, two things were fixed:

* The entirety of #14451.
* One of the test cases in #7503. I've added this as T7503a. The
  other test case from that ticket still does /not/ work, so we'll
  have to add T7503b some other day.

testsuite/tests/typecheck/should_compile/T14451.hs

+{-# Language KindSignatures, TypeOperators, PolyKinds, TypeOperators, ConstraintKinds, TypeFamilies, DataKinds, TypeInType, GADTs, AllowAmbiguousTypes, InstanceSigs, RankNTypes, UndecidableInstances #-}
+module T14451 where
+
+import Data.Kind
+
+data TyFun :: Type -> Type -> Type
+
+type a ~> b = TyFun a b -> Type
+
+type Cat ob = ob -> ob -> Type
+
+type family
+  Apply (f :: a ~> b) (x :: a) :: b where
+  Apply (CompSym2 f g) a = Comp f g a
+
+data CompSym2 :: (b ~> c) -> (a ~> b) -> (a ~> c)
+
+type a·b = Apply a b
+
+class Varpi (f :: i ~> j) where
+  type Dom (f :: i ~> j) :: Cat i
+  type Cod (f :: i ~> j) :: Cat j
+
+  varpa :: Dom f a a' -> Cod f (f·a) (f·a')
+
+type family
+  Comp (f::k1 ~> k) (g::k2 ~> k1) (a::k2) :: k where
+  Comp f g a = f · (g · a)

testsuite/tests/typecheck/should_compile/T7503a.hs

+{-# LANGUAGE ExistentialQuantification, DataKinds, PolyKinds, KindSignatures, GADTs #-}
+module T7503a where
+    import Data.Kind
+    import GHC.Exts hiding (Any)
+
+    data WrappedType = forall a. WrapType a
+
+    data A :: WrappedType -> Type where
+        MkA :: forall (a :: Type). AW a -> A (WrapType a)
+
+    type AW  (a :: k) = A (WrapType a)
+    type AW' (a :: k) = A (WrapType a)
+
+    class C (a :: k) where
+        aw :: AW a -- workaround: AW'
+
+    instance C [] where
+        aw = aw

testsuite/tests/typecheck/should_compile/all.T

@@ -401,6 +401,7 @@ test('type_in_type_hole_fits', normal, compile, ['-fdefer-type-errors -fno-max-v
 test('T15370', normal, compile, ['-fdefer-type-errors -fno-max-valid-hole-fits -funclutter-valid-hole-fits'])
 test('T7408', normal, compile, [''])
 test('UnboxStrictPrimitiveFields', normal, compile, [''])
+test('T7503a', normal, compile, [''])
 test('T7541', normal, compile, [''])
 test('T7562', normal, compile, [''])
 test('T7641', normal, compile, [''])
@@ -596,6 +597,7 @@ test('T14396', [extra_files(['T14396.hs', 'T14396.hs-boot', 'T14396a.hs', 'T1439
 test('T14434', [], run_command, ['$MAKE -s --no-print-directory T14434'])
 test('MissingExportList01', normal, compile, [''])
 test('MissingExportList02', normal, compile, [''])
+test('T14451', normal, compile, [''])
 test('T14488', normal, compile, [''])
 test('T14590', normal, compile, ['-fdefer-type-errors -fno-max-valid-hole-fits'])
 # We omit the hpc/profasm ways because this test checks the