test we reject equivalent signatures with more than one argument

src/test/compile-fail/impl-trait/impl-generic-mismatch-ab.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 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(universal_impl_trait)]
+use std::fmt::Debug;
+
+trait Foo {
+    fn foo<A: Debug>(&self, a: &A, b: &impl Debug);
+}
+
+impl Foo for () {
+    fn foo<B: Debug>(&self, a: &impl Debug, b: &B) { }
+    //~^ ERROR method `foo` has an incompatible type for trait
+}
+
+fn main() {}