Fallback: Tests to clarify defaults for Self in trait impls.

Author: leoyvens

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

@@ -0,0 +1,38 @@
+// 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)]
+
+use std::fmt::Debug;
+
+trait B {
+    fn b(&self) -> Self;
+}
+
+// Strangely, having a default for `Self`
+// will fail for fundamental types such as `T` or `Box<T>`.
+// Replace `Option` for `Box` in this test to reproduce.
+impl<T=String> B for Option<T>
+    where T: Default
+{
+    fn b(&self) -> Option<T> {
+        Some(T::default())
+    }
+}
+
+fn main() {
+    let x = None;
+    foo(x.b());
+}
+
+fn foo<T=String>(a: Option<T>)
+    where T: Debug + PartialEq<&'static str> {
+    assert_eq!(a.unwrap(), "");
+}

src/test/ui/default-ty-param-fallback/trait_method_call_default_self.rs

@@ -0,0 +1,36 @@
+// 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.
+// compile-flags: --error-format=human
+
+#![feature(default_type_parameter_fallback)]
+
+use std::fmt::Debug;
+
+trait B {
+    fn b(&self) -> Self;
+}
+
+impl<T=String> B for Option<T>
+    where T: Default
+{
+    fn b(&self) -> Option<T> {
+        Some(T::default())
+    }
+}
+
+fn main() {
+    let x = None;
+    foo(x.b());
+}
+
+fn foo<T=i32>(a: Option<T>)
+    where T: Debug + PartialEq<&'static str> {
+    assert_eq!(a.unwrap(), "");
+}

src/test/ui/default-ty-param-fallback/trait_method_call_default_self.stderr

@@ -0,0 +1,10 @@
+error[E0282]: type annotations needed
+  --> $DIR/trait_method_call_default_self.rs:29:13
+   |
+29 |     let x = None;
+   |         -   ^^^^ cannot infer type for `T`
+   |         |
+   |         consider giving `x` a type
+
+error: aborting due to previous error
+