Give where clauses priority over builtin rules. Fixes #20959.

src/librustc/middle/traits/select.rs

@@ -1166,6 +1166,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         .is_ok()
                 })
             }
+            (&BuiltinCandidate(_), &ParamCandidate(_)) => {
+                // If we have a where-clause like `Option<K> : Send`,
+                // then we wind up in a situation where there is a
+                // default rule (`Option<K>:Send if K:Send) and the
+                // where-clause that both seem applicable. Just take
+                // the where-clause in that case.
+                true
+            }
             (&ProjectionCandidate, &ParamCandidate(_)) => {
                 // FIXME(#20297) -- this gives where clauses precedent
                 // over projections. Really these are just two means

src/test/run-pass/trait-false-ambiguity-where-clause-builtin-bound.rs

@@ -0,0 +1,23 @@
+// Copyright 2015 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.
+
+// Test that we do not error out because of a (False) ambiguity
+// between the builtin rules for Sized and the where clause. Issue
+// #20959.
+
+fn foo<K>(x: Option<K>)
+    where Option<K> : Sized
+{
+    let _y = x;
+}
+
+fn main() {
+    foo(Some(22));
+}