diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index cf30969ebef..fb216a5e99d 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -2604,12 +2604,17 @@ impl FlagComputation {
 
             &ty_projection(ref data) => {
                 self.add_flags(HAS_PROJECTION);
-                self.add_substs(data.trait_ref.substs);
+                self.add_projection_ty(data);
             }
 
             &ty_trait(box TyTrait { ref principal, ref bounds }) => {
                 let mut computation = FlagComputation::new();
                 computation.add_substs(principal.0.substs);
+                for projection_bound in bounds.projection_bounds.iter() {
+                    let mut proj_computation = FlagComputation::new();
+                    proj_computation.add_projection_predicate(&projection_bound.0);
+                    computation.add_bound_computation(&proj_computation);
+                }
                 self.add_bound_computation(&computation);
 
                 self.add_bounds(bounds);
@@ -2673,6 +2678,15 @@ impl FlagComputation {
         }
     }
 
+    fn add_projection_predicate(&mut self, projection_predicate: &ProjectionPredicate) {
+        self.add_projection_ty(&projection_predicate.projection_ty);
+        self.add_ty(projection_predicate.ty);
+    }
+
+    fn add_projection_ty(&mut self, projection_ty: &ProjectionTy) {
+        self.add_substs(projection_ty.trait_ref.substs);
+    }
+
     fn add_substs(&mut self, substs: &Substs) {
         self.add_tys(substs.types.as_slice());
         match substs.regions {
diff --git a/src/test/run-pass/associated-types-projection-in-object-type.rs b/src/test/run-pass/associated-types-projection-in-object-type.rs
new file mode 100644
index 00000000000..44dd49b7297
--- /dev/null
+++ b/src/test/run-pass/associated-types-projection-in-object-type.rs
@@ -0,0 +1,43 @@
+// Copyright 2015 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.
+
+// Corrected regression test for #20831. The original did not compile.
+// When fixed, it revealed another problem concerning projections that
+// appear in associated type bindings in object types, which were not
+// being properly flagged.
+
+use std::ops::{Shl, Shr};
+use std::cell::RefCell;
+
+pub trait Subscriber {
+    type Input;
+}
+
+pub trait Publisher<'a> {
+    type Output;
+    fn subscribe(&mut self, Box<Subscriber<Input=Self::Output> + 'a>);
+}
+
+pub trait Processor<'a> : Subscriber + Publisher<'a> { }
+
+impl<'a, P> Processor<'a> for P where P : Subscriber + Publisher<'a> { }
+
+struct MyStruct<'a> {
+    sub: Box<Subscriber<Input=u64> + 'a>
+}
+
+impl<'a> Publisher<'a> for MyStruct<'a> {
+    type Output = u64;
+    fn subscribe(&mut self, t : Box<Subscriber<Input=u64> + 'a>) {
+        self.sub = t;
+    }
+}
+
+fn main() {}