error on any use of trait alias

src/librustc_typeck/collect.rs

@@ -442,9 +442,8 @@ fn convert_item<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item_id: ast::NodeId) {
             tcx.predicates_of(def_id);
         },
         hir::ItemTraitAlias(..) => {
-            tcx.generics_of(def_id);
+            span_err!(tcx.sess, it.span, E0645,
-            tcx.trait_def(def_id);
+                      "trait aliases are not yet implemented (see issue #41517)");
-            tcx.predicates_of(def_id);
         },
         hir::ItemStruct(ref struct_def, _) |
         hir::ItemUnion(ref struct_def, _) => {

src/librustc_typeck/diagnostics.rs

@@ -4721,4 +4721,5 @@ register_diagnostics! {
     E0632, // cannot provide explicit type parameters when `impl Trait` is used in
            // argument position.
     E0641, // cannot cast to/from a pointer with an unknown kind
+    E0645, // trait aliases not finished
 }

src/test/compile-fail/trait-alias.rs

@@ -9,9 +9,11 @@
 // except according to those terms.
 
 trait Alias1<T> = Default where T: Clone; // ok
+    //~^ERROR trait aliases are not yet implemented
 trait Alias2<T: Clone = ()> = Default;
     //~^ERROR type parameters on the left side of a trait alias cannot be bounded
     //~^^ERROR type parameters on the left side of a trait alias cannot have defaults
+    //~^^^ERROR trait aliases are not yet implemented
 
 impl Alias1 { //~ERROR expected type, found trait alias
     fn foo() {}

src/test/ui/trait-alias.rs

@@ -8,9 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-trait SimpleAlias = Default;
+trait SimpleAlias = Default; //~ERROR E0645
-trait GenericAlias<T> = Iterator<Item=T>;
+trait GenericAlias<T> = Iterator<Item=T>; //~ERROR E0645
-trait Partial<T> = IntoIterator<Item=T>;
+trait Partial<T> = IntoIterator<Item=T>; //~ERROR E0645
 
 trait Things<T> {}
 trait Romeo {}
@@ -19,10 +19,10 @@ struct Fore<T>(T);
 impl<T, U> Things<T> for The<U> {}
 impl<T> Romeo for Fore<T> {}
 
-trait WithWhere<Art, Thou> = Romeo + Romeo where Fore<(Art, Thou)>: Romeo;
+trait WithWhere<Art, Thou> = Romeo + Romeo where Fore<(Art, Thou)>: Romeo; //~ERROR E0645
-trait BareWhere<Wild, Are> = where The<Wild>: Things<Are>;
+trait BareWhere<Wild, Are> = where The<Wild>: Things<Are>; //~ERROR E0645
 
-trait CD = Clone + Default;
+trait CD = Clone + Default; //~ERROR E0645
 
 fn foo<T: CD>() -> (T, T) {
     let one = T::default();

src/test/ui/trait-alias.stderr

@@ -0,0 +1,38 @@
+error[E0645]: trait aliases are not yet implemented (see issue #41517)
+  --> $DIR/trait-alias.rs:11:1
+   |
+11 | trait SimpleAlias = Default; //~ERROR E0645
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0645]: trait aliases are not yet implemented (see issue #41517)
+  --> $DIR/trait-alias.rs:12:1
+   |
+12 | trait GenericAlias<T> = Iterator<Item=T>; //~ERROR E0645
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0645]: trait aliases are not yet implemented (see issue #41517)
+  --> $DIR/trait-alias.rs:13:1
+   |
+13 | trait Partial<T> = IntoIterator<Item=T>; //~ERROR E0645
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0645]: trait aliases are not yet implemented (see issue #41517)
+  --> $DIR/trait-alias.rs:22:1
+   |
+22 | trait WithWhere<Art, Thou> = Romeo + Romeo where Fore<(Art, Thou)>: Romeo; //~ERROR E0645
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0645]: trait aliases are not yet implemented (see issue #41517)
+  --> $DIR/trait-alias.rs:23:1
+   |
+23 | trait BareWhere<Wild, Are> = where The<Wild>: Things<Are>; //~ERROR E0645
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0645]: trait aliases are not yet implemented (see issue #41517)
+  --> $DIR/trait-alias.rs:25:1
+   |
+25 | trait CD = Clone + Default; //~ERROR E0645
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+