Test that trait definition owns the default, not the impl.

I don't know if it's what we want but it's how it works now. If we want
to keep it then we should forbid writing the default in the impl.

src/test/run-pass/default-ty-param-fallback/trait_def_rules_over_impl.rs

@@ -0,0 +1,31 @@
+// 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.
+
+#![feature(default_type_parameter_fallback)]
+
+trait Bar {
+    // Current we consider only the default in the trait,
+    // never the one in the impl.
+    // Is this good? Is it bad? Is it just the way it works?
+    // If this is ok then we should forbid writing the default in the impl.
+    fn method<A:Default=String>(&self) -> A;
+}
+
+struct Foo;
+
+impl Bar for Foo {
+    fn method<A:Default>(&self) -> A {
+        A::default()
+    }
+}
+
+fn main() {
+    Foo.method();
+}