Don't use skolemized parameters but rather fresh variables in

coherence. Skolemized parameters wind up preventing unification.
Surprised we had no test for this! Fixes #24241.

src/test/compile-fail/coherence-overlap-all-t-and-tuple.rs

@@ -0,0 +1,28 @@
+// 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.
+
+// Check that we detect an overlap here in the case where:
+//
+//    for some type X:
+//      T = (X,)
+//      T11 = X, U11 = X
+//
+// Seems pretty basic, but then there was issue #24241. :)
+
+trait From<U> {
+}
+
+impl <T> From<T> for T { //~ ERROR E0119
+}
+
+impl <T11, U11> From<(U11,)> for (T11,) {
+}
+
+fn main() { }