Update(orderBy): Guaranteed stable sort

Falls back on built-in comparison function if user-provided comparison function
fails to break a tie between elements being ordered.

Closes #14881

Author: BobChao87

src/ng/filter/orderBy.js

@@ -459,6 +459,9 @@
  * comparator function. For example, you might need to compare some strings in a locale-sensitive
  * way. (When specifying a custom comparator, you also need to pass a value for the `reverse`
  * argument - passing `false` retains the default sorting order, i.e. ascending.)
+ *
+ * If your comparator fails to differentiate between two items, they will fall back to the built-in
+ * comparator function, this produces a guarantee of stable sorting.
  *
    <example name="orderBy-custom-comparator" module="orderByExample4">
      <file name="index.html">
@@ -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() {