deprecating types after colon

compiler/testData/diagnostics/tests/cast/bare/EitherSafeAs.kt

@@ -1,3 +1,5 @@
+// !CHECK_TYPE
+
 trait Either<out A, out B>
 trait Left<out A>: Either<A, Nothing>
 trait Right<out B>: Either<Nothing, B>
@@ -7,10 +9,10 @@ class C2(val v2: Int)
 
 fun _as_left(e: Either<C1, C2>): Any? {
     val v = e as? Left
-    return v: Left<C1>?
+    return checkSubtype<Left<C1>?>(v)
 }
 
 fun _as_right(e: Either<C1, C2>): Any? {
     val v = e as? Right
-    return v: Right<C2>?
+    return checkSubtype<Right<C2>?>(v)
 }