Concrete methods cannot be overridden by abstract methods

lib/member_builder.js

@@ -76,6 +76,15 @@ exports.buildMethod = function( members, meta, name, value, keywords )
             );
         }
 
+        // do not allow overriding concrete methods with abstract
+        if ( keywords[ 'abstract' ] && !( util.isAbstractMethod( prev ) ) )
+        {
+            throw TypeError(
+                "Cannot override concrete method '" + name + "' with " +
+                    "abstract method"
+            );
+        }
+
         // ensure parameter list is at least the length of its supertype
         if ( ( value.__length || value.length )
             < ( prev.__length || prev.length )

test/test-member_builder-method.js

@@ -134,3 +134,20 @@ mb_common.assertCommon();
     mb_common.members[ 'public' ][ mb_common.name ]();
 } )();
 
+
+/**
+ * Once a concrete implementation has been defined for a method, a subtype
+ * cannot make it abstract.
+ */
+( function testCannotOverrideConcreteMethodWithAbstractMethod()
+{
+    // concrete method
+    mb_common.value = function() {};
+    mb_common.buildMemberQuick();
+
+    assert.throws( function()
+    {
+        mb_common.buildMemberQuick( { 'abstract': true }, true );
+    }, TypeError, "Cannot override concrete method with abstract method" );
+} )();
+