Fix associated existentials for generic traits

src/librustc/traits/project.rs

@@ -1517,7 +1517,8 @@ fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
     }
     let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
     let ty = if let ty::AssociatedKind::Existential = assoc_ty.item.kind {
-        tcx.mk_anon(assoc_ty.item.def_id, substs)
+        let item_substs = Substs::identity_for_item(tcx, assoc_ty.item.def_id);
+        tcx.mk_anon(assoc_ty.item.def_id, item_substs)
     } else {
         tcx.type_of(assoc_ty.item.def_id)
     };

src/test/ui/impl-trait/associated-existential-type-generic-trait.rs

@@ -0,0 +1,40 @@
+// 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(existential_type)]
+// compile-pass
+
+trait Bar {}
+struct Dummy<U>(U);
+impl<V> Bar for Dummy<V> {}
+
+trait Foo<T> {
+    type Assoc: Bar;
+    fn foo(t: T) -> Self::Assoc;
+}
+
+impl<W> Foo<W> for i32 {
+    existential type Assoc: Bar;
+    fn foo(w: W) -> Self::Assoc {
+        Dummy(w)
+    }
+}
+
+struct NonGeneric;
+impl Bar for NonGeneric {}
+
+impl<W> Foo<W> for u32 {
+    existential type Assoc: Bar;
+    fn foo(_: W) -> Self::Assoc {
+        NonGeneric
+    }
+}
+
+fn main() {}