Test Trac #8856

(cherry picked from commit 062391be)

testsuite/tests/typecheck/should_compile/T8856.hs

+{-# LANGUAGE ScopedTypeVariables, RankNTypes, PolyKinds #-}
+module T8856 where
+
+import Data.Proxy
+
+foo = (undefined :: Proxy a) :: forall a. Proxy a

testsuite/tests/typecheck/should_compile/all.T

@@ -417,3 +417,4 @@ test('T8565', normal, compile, [''])
 test('T8644', normal, compile, [''])
 test('T8762', normal, compile, [''])
 test('MutRec', normal, compile, [''])
+test('T8856', normal, compile, [''])