Added WF checking for trait alias definitions.

Author: Alexander Regueiro

src/librustc_typeck/check/wfcheck.rs

@@ -153,6 +153,9 @@ pub fn check_item_well_formed<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: Def
         hir::ItemKind::Trait(..) => {
             check_trait(tcx, item);
         }
+        hir::ItemKind::TraitAlias(..) => {
+            check_trait(tcx, item);
+        }
         _ => {}
     }
 }

src/test/ui/traits/trait-alias-fail1.stderr

@@ -0,0 +1,17 @@
+// Copyright 2018 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(trait_alias)]
+
+trait DefaultAlias = Default;
+
+impl DefaultAlias for () {}
+
+fn main() {}

src/test/ui/traits/trait-alias-fail2.stderr

@@ -0,0 +1,9 @@
+error[E0404]: expected trait, found trait alias `DefaultAlias`
+  --> $DIR/trait-alias-impl.rs:15:6
+   |
+LL | impl DefaultAlias for () {}
+   |      ^^^^^^^^^^^^ not a trait
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0404`.

src/test/ui/traits/trait-alias-impl.rs

@@ -0,0 +1,18 @@
+error[E0038]: the trait `EqAlias` cannot be made into an object
+  --> $DIR/trait-alias-objects.rs:17:13
+   |
+LL |     let _: &dyn EqAlias = &123;
+   |             ^^^^^^^^^^^ the trait `EqAlias` cannot be made into an object
+   |
+   = note: the trait cannot use `Self` as a type parameter in the supertraits or where-clauses
+
+error[E0191]: the value of the associated type `Item` (from the trait `std::iter::Iterator`) must be specified
+  --> $DIR/trait-alias-objects.rs:18:13
+   |
+LL |     let _: &dyn IteratorAlias = &vec![123].into_iter();
+   |             ^^^^^^^^^^^^^^^^^ missing associated type `Item` value
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0038, E0191.
+For more information about an error, try `rustc --explain E0038`.

src/test/ui/traits/trait-alias-impl.stderr

@@ -1,4 +1,4 @@
-// Copyright 2017-2018 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -10,13 +10,8 @@
 
 #![feature(trait_alias)]
 
-trait CloneDefault<T> = Default where T: Clone;
-trait BoundedAlias<T: Clone = ()> = Default;
-
 trait Foo {}
 trait A<T: Foo> {}
 trait B<T> = A<T>; // T cannot be unbounded
 
-impl CloneDefault for () {}
-
 fn main() {}

src/test/ui/traits/trait-alias-fail2.rs renamed to src/test/ui/traits/trait-alias-objects.rs

@@ -0,0 +1,16 @@
+error[E0277]: the trait bound `T: Foo` is not satisfied
+  --> $DIR/trait-alias-wf.rs:15:1
+   |
+LL | trait B<T> = A<T>; // T cannot be unbounded
+   | ^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `T`
+   |
+   = help: consider adding a `where T: Foo` bound
+note: required by `A`
+  --> $DIR/trait-alias-wf.rs:14:1
+   |
+LL | trait A<T: Foo> {}
+   | ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.