Refactor tests.

Make test more focused, and clarify what the bug is. Add new ui test.

Author: leoyvens

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

@@ -0,0 +1,40 @@
+// 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 B<T> {
+    fn b(&self) -> T;
+}
+
+impl<T = X> B<T> for X
+    where T: Default
+{
+    fn b(&self) -> T {
+        T::default()
+    }
+}
+
+#[derive(Copy, Clone, Default)]
+struct X(u8);
+
+fn main() {
+    let x = X(0);
+    let y = x.b();
+    foo(y);
+    // Uncommenting the following line makes the fallback fail.
+    // Probably we are creating a new inference var,
+    // that is carrying over the origin but not the value of the default.
+    // x.0;
+}
+
+fn foo<T = X>(a: T) -> T {
+    a
+}

src/test/run-pass/default-ty-param-fallback/trait_impl_with_param_in_trait.rs renamed to src/test/ui/default-ty-param-fallback/trait_impl_param_in_trait.rs

@@ -7,39 +7,31 @@
 // <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)]
 
-trait A<T = Self> {
-    fn a(t: &T) -> Self;
-}
-
-trait B<T = Self> {
+trait B<T> {
     fn b(&self) -> T;
 }
 
 impl<T = X> B<T> for X
-    where T: A<X>
+    where T: Default
 {
     fn b(&self) -> T {
-        T::a(self)
+        T::default()
     }
 }
 
+#[derive(Copy, Clone, Default)]
 struct X(u8);
 
-impl A for X {
-    fn a(x: &X) -> X {
-        X(x.0)
-    }
-}
-
-fn g(x: &X) {
-    // Bug: replacing with `let x = X::b(x)` will cause `x` to be unknown in `x.0`.
-    let x = <X as B>::b(x);
-    x.0;
+fn main() {
+    let x = X(0);
+    let y = x.b();
+    foo(y);
 }
 
-fn main() {
-    g(&X(0));
+fn foo<T = u32>(a: T) -> T {
+    a
 }

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

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed
+  --> $DIR/trait_impl_param_in_trait.rs:31:9
+   |
+31 |     let y = x.b();
+   |         ^
+   |         |
+   |         cannot infer type for `_`
+   |         consider giving `y` a type
+
+error: aborting due to previous error
+