Fix Ppat_cons parenthesization in function parameter position (#2476)

Author: mdelvecchio-jsc

lib/Ast.ml

@@ -1890,6 +1890,7 @@ end = struct
       | Ppat_cons _ -> true
       | Ppat_construct _ | Ppat_record _ | Ppat_variant _ -> false
       | _ -> true )
+    | Fp {pparam_desc= Pparam_val (_, _, _); _}, Ppat_cons _ -> true
     | Pat {ppat_desc= Ppat_construct _; _}, Ppat_cons _ -> true
     | _, Ppat_constraint (_, {ptyp_desc= Ptyp_poly _; _}) -> false
     | ( Exp {pexp_desc= Pexp_letop _; _}

test/passing/tests/apply.ml

@@ -83,3 +83,7 @@ let _ =
     (loooooooooooong looooooooooooooong loooooooooooooong
        [loooooooooong; loooooooooooong; loooooooooooooooooooooong]
     )
+
+let f (x :: y) = x
+
+let f (* xx *) ((* aa *) x (* bb *) :: (* cc *) y (* dd *)) (* yy *) = x