fix(orderBy): guarantee stable sort

If a user-provided comparator fails to differentiate between two items, fall
back to the built-in comparator (using the tie-breaker predicate).

Fixes #14881

Closes #15914

src/ng/filter/orderBy.js

@@ -53,6 +53,9 @@
  * dummy predicate that returns the item's index as `value`.
  * (If you are using a custom comparator, make sure it can handle this predicate as well.)
  *
+ * If a custom comparator still can't distinguish between two items, then they will be sorted based
+ * on their index using the built-in comparator.
+ *
  * Finally, in an attempt to simplify things, if a predicate returns an object as the extracted
  * value for an item, `orderBy` will try to convert that object to a primitive value, before passing
  * it to the comparator. The following rules govern the conversion:
@@ -599,7 +602,7 @@ function orderByFilter($parse) {
         }
       }
 
-      return compare(v1.tieBreaker, v2.tieBreaker) * descending;
+      return (compare(v1.tieBreaker, v2.tieBreaker) || defaultCompare(v1.tieBreaker, v2.tieBreaker)) * descending;
     }
   };
 

test/ng/filter/orderBySpec.js

@@ -457,6 +457,19 @@ describe('Filter: orderBy', function() {
 
         expect(orderBy(items, expr, reverse, comparator)).toEqual(sorted);
       });
+
+      it('should use the default comparator to break ties on a provided comparator', function() {
+        // Some list that won't be sorted "naturally", i.e. should sort to ['a', 'B', 'c']
+        var items = ['c', 'a', 'B'];
+        var expr = null;
+        function comparator() {
+          return 0;
+        }
+        var reversed = ['B', 'a', 'c'];
+
+        expect(orderBy(items, expr, false, comparator)).toEqual(items);
+        expect(orderBy(items, expr, true, comparator)).toEqual(reversed);
+      });
     });
 
     describe('(object as `value`)', function() {