Catch attempts to project nonexistent associated items from a trait.

Fix rust-lang#22731.

src/libcore/iter.rs

@@ -978,7 +978,7 @@ pub trait IteratorExt: Iterator + Sized {
     #[unstable(feature = "core", reason = "recent addition")]
     fn cloned(self) -> Cloned<Self> where
         Self::Item: Deref,
-        <Self::Item as Deref>::Output: Clone,
+        <Self::Item as Deref>::Target: Clone,
     {
         Cloned { it: self }
     }

src/librustc_typeck/collect.rs

@@ -244,12 +244,20 @@ impl<'a, 'tcx> AstConv<'tcx> for CollectCtxt<'a, 'tcx> {
     }
 
     fn projected_ty(&self,
-                    _span: Span,
+                    span: Span,
                     trait_ref: Rc<ty::TraitRef<'tcx>>,
                     item_name: ast::Name)
                     -> Ty<'tcx>
     {
-        ty::mk_projection(self.tcx(), trait_ref, item_name)
+        let trait_def = ty::lookup_trait_def(self.tcx, trait_ref.def_id);
+        if !trait_def.associated_type_names.contains(&item_name) {
+            span_err!(self.tcx.sess, span, E0320,
+                      "cannot find binding for item `{}` within trait `{}`",
+                      item_name.as_str(), trait_ref.user_string(self.tcx));
+            ty::mk_t(self.tcx(), ty::ty_err)
+        } else {
+            ty::mk_projection(self.tcx(), trait_ref, item_name)
+        }
     }
 }
 

src/librustc_typeck/diagnostics.rs

@@ -173,7 +173,8 @@ register_diagnostics! {
     E0249, // expected constant expr for array length
     E0250, // expected constant expr for array length
     E0318, // can't create default impls for traits outside their crates
-    E0319  // trait impls for defaulted traits allowed just for structs/enums
+    E0319, // trait impls for defaulted traits allowed just for structs/enums
+    E0320  // Trait does not provide binding for Type in `<T as Trait>::Type`
 }
 
 __build_diagnostic_array! { DIAGNOSTICS }

src/test/compile-fail/issue-19883.rs

@@ -18,13 +18,11 @@ trait To {
     // This is a typo, the return type should be `<Dst as From<Self>>::Output`
     fn to<Dst: From<Self>>(
         self
-        //~^ error: the trait `core::marker::Sized` is not implemented
     ) ->
         <Dst as From<Self>>::Dst
-        //~^ error: the trait `core::marker::Sized` is not implemented
+        //~^ error: cannot find binding for item `Dst` within trait `From<Self>`
     {
         From::from(
-            //~^ error: the trait `core::marker::Sized` is not implemented
             self
         )
     }