test: Add test for fixed issue #12796

Doesn't close #12796 because the error message is awful.

src/test/compile-fail/issue-12796.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 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.
+
+// error-pattern: missing `Self` type param in the substitution of `fn(Self)`
+
+trait Trait {
+    fn outer(self) {
+        fn inner(_: Self) {
+        }
+    }
+}
+
+fn main() { }