remove wrong assert in check_match

the assert was invalidated by PR rust-lang#31020

Fixes rust-lang#31561

src/librustc/middle/check_match.rs

@@ -1047,10 +1047,7 @@ fn is_refutable<A, F>(cx: &MatchCheckCtxt, pat: &Pat, refutable: F) -> Option<A>
 {
     let pats = Matrix(vec!(vec!(pat)));
     match is_useful(cx, &pats, &[DUMMY_WILD_PAT], ConstructWitness) {
-        UsefulWithWitness(pats) => {
-            assert_eq!(pats.len(), 1);
-            Some(refutable(&pats[0]))
-        },
+        UsefulWithWitness(pats) => Some(refutable(&pats[0])),
         NotUseful => None,
         Useful => unreachable!()
     }

src/test/compile-fail/issue-31561.rs

@@ -0,0 +1,20 @@
+// Copyright 2016 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.
+
+enum Thing {
+    Foo(u8),
+    Bar,
+    Baz
+}
+
+fn main() {
+    let Thing::Foo(y) = Thing::Foo(1);
+    //~^ ERROR refutable pattern in local binding: `Bar` not covered
+}