Use T as the subpattern type of Box<T>

The subpattern type of boxes being nil does not make sense because of
box patterns. They should have their inner type as the subpattern type.

src/test/run-pass/issue-42679.rs

@@ -0,0 +1,31 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(box_syntax)]
+#![feature(box_patterns)]
+
+#[derive(Debug, PartialEq)]
+enum Test {
+    Foo(usize),
+    Bar(isize),
+}
+
+fn main() {
+    let a = box Test::Foo(10);
+    let b = box Test::Bar(-20);
+    match (a, b) {
+        (_, box Test::Foo(_)) => unreachable!(),
+        (box Test::Foo(x), b) => {
+            assert_eq!(x, 10);
+            assert_eq!(b, box Test::Bar(-20));
+        },
+        _ => unreachable!(),
+    }
+}