[FIXED JENKINS-18771]

... with a test case. The fix is in Stapler.

changelog.html

@@ -58,6 +58,9 @@ Upcoming changes</a>
   <li class=bug>
     Provided maven settings.xml in maven builder is lost.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-15976">issue 15976</a>)
+  <li class=bug>
+    Fixed a regression that broke some plugins' form validation
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-18776">issue 18776</a>)
 </ul>
 </div><!--=TRUNK-END=-->
 

core/pom.xml

@@ -42,7 +42,7 @@ THE SOFTWARE.
 
   <properties>
     <staplerFork>true</staplerFork>
-    <stapler.version>1.213</stapler.version>
+    <stapler.version>1.214</stapler.version>
     <spring.version>2.5.6.SEC03</spring.version>
   </properties>
 

test/pom.xml

@@ -193,6 +193,28 @@ THE SOFTWARE.
           <redirectTestOutputToFile>true</redirectTestOutputToFile>
         </configuration>
       </plugin>
+      <plugin>
+        <groupId>org.codehaus.gmaven</groupId>
+        <artifactId>gmaven-plugin</artifactId>
+        <!-- version specified in grandparent pom -->
+        <executions>
+          <execution>
+            <id>test-in-groovy</id>
+            <!-- compile and process annotations in Groovy test code -->
+            <goals>
+              <goal>generateTestStubs</goal>
+              <goal>testCompile</goal>
+            </goals>
+          </execution>
+        </executions>
+        <dependencies>
+          <dependency>
+            <groupId>ant</groupId>
+            <artifactId>ant</artifactId>
+            <version>1.6.5</version>
+          </dependency>
+        </dependencies>
+      </plugin>
       <plugin>
         <groupId>org.kohsuke.gmaven</groupId>
         <artifactId>gmaven-plugin</artifactId>
@@ -208,13 +230,6 @@ THE SOFTWARE.
               <source>${pom.basedir}/src/main/preset-data/package.groovy</source>
             </configuration>
           </execution>
-          <execution>
-            <id>test-in-groovy</id>
-            <!-- allow tests written in Groovy -->
-            <goals>
-              <goal>testCompile</goal>
-            </goals>
-          </execution>
         </executions>
         <dependencies>
           <dependency>

test/src/test/groovy/hudson/RelativePathTest.groovy

+package hudson
+
+import hudson.model.AbstractDescribableImpl
+import hudson.model.Describable
+import hudson.model.Descriptor
+import hudson.util.ListBoxModel
+import org.jvnet.hudson.test.Bug
+import org.jvnet.hudson.test.HudsonTestCase
+import org.jvnet.hudson.test.TestExtension
+import org.kohsuke.stapler.QueryParameter
+
+/**
+ * Regression test for JENKINS-18776
+ *
+ * @author Kohsuke Kawaguchi
+ */
+class RelativePathTest extends HudsonTestCase implements Describable<RelativePathTest> {
+    @Bug(18776)
+    void testRelativePath() {
+        // I was having trouble causing annotation processing on test stubs
+        jenkins.getDescriptorOrDie(RelativePathTest.class)
+        jenkins.getDescriptorOrDie(Model.class)
+
+        createWebClient().goTo("/self/");
+        assert jenkins.getDescriptorOrDie(Model.class).touched
+    }
+
+    String getName() {
+        return "Alice";
+    }
+
+    Model getModel() {
+        return new Model();
+    }
+
+    DescriptorImpl getDescriptor() {
+        return jenkins.getDescriptorOrDie(getClass());
+    }
+
+    @TestExtension
+    static class DescriptorImpl extends Descriptor<RelativePathTest> {
+        @Override
+        String getDisplayName() {
+            return "";
+        }
+    }
+
+    static class Model extends AbstractDescribableImpl<Model> {
+        @TestExtension
+        static class DescriptorImpl extends Descriptor<Model> {
+            boolean touched;
+
+            @Override
+            String getDisplayName() {
+                return "test";
+            }
+
+            ListBoxModel doFillAbcItems(@RelativePath("..") @QueryParameter String name) {
+                assert name=="Alice";
+                touched = true;
+                return new ListBoxModel().add("foo").add("bar")
+            }
+        }
+    }
+}

test/src/test/resources/hudson/RelativePathTest/Model/config.groovy

+def f= namespace(lib.FormTagLib)
+
+f.entry(field:"abc") {
+    f.select()
+}
\ No newline at end of file

test/src/test/resources/hudson/RelativePathTest/index.groovy

+package hudson.RelativePathTest
+
+def l = namespace(lib.LayoutTagLib)
+def j = namespace("jelly:core")
+def f= namespace(lib.FormTagLib)
+
+l.layout {
+    l.main_panel {
+        set("instance",my)
+        set("descriptor",my.descriptor)
+        f.form() {
+            f.entry(field:"name") {
+                f.textbox()
+            }
+            f.property(field:"model")
+        }
+    }
+}