Only check predicates for late-bound non-lifetime vars in object candidate assembly

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -628,7 +628,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
 
         self.infcx.probe(|_snapshot| {
-            if obligation.has_non_region_late_bound() {
+            // FIXME(non_lifetime_binders): Really, we care only to check that we don't
+            // have any non-region *escaping* bound vars, but that's hard to check and
+            // we shouldn't really ever encounter those anyways.
+            if obligation.self_ty().has_non_region_late_bound() {
                 return;
             }
 

tests/ui/traits/non_lifetime_binders/disqualifying-object-candidates.rs

@@ -0,0 +1,19 @@
+// check-pass
+
+trait Foo {
+    type Bar<T>
+    where
+        dyn Send + 'static: Send;
+}
+
+impl Foo for () {
+    type Bar<T> = i32;
+    // We take `<() as Foo>::Bar<T>: Sized` and normalize it under the where clause
+    // of `for<S> <() as Foo>::Bar<S> = i32`. This gives us back `i32: Send` with
+    // the nested obligation `(dyn Send + 'static): Send`. However, during candidate
+    // assembly for object types, we disqualify any obligations that has non-region
+    // late-bound vars in the param env(!), rather than just the predicate. This causes
+    // the where clause to not hold even though it trivially should.
+}
+
+fn main() {}