Warn when the aggregate goal skips a non-pom project

src/main/java/org/apache/maven/plugins/source/AggregatorSourceJarMojo.java

@@ -28,16 +28,23 @@
  *
  * @since 2.0.3
  */
-@Mojo(name = "aggregate", defaultPhase = "package", aggregator = true)
+@Mojo(name = AggregatorSourceJarMojo.AGGREGATE_GOAL, defaultPhase = "package", aggregator = true)
 @Execute(phase = "generate-sources")
 public class AggregatorSourceJarMojo extends SourceJarMojo {
+
+    static final String AGGREGATE_GOAL = "aggregate";
+
     /**
      * {@inheritDoc}
      */
     @Override
     protected void doExecute() throws MojoException {
-        if (Type.POM.equals(getProject().getPackaging().type().id())) {
+        String packaging = getProject().getPackaging().type().id();
+        if (Type.POM.equals(packaging)) {
             packageSources(reactorProjects);
+        } else {
+            getLog().warn("Not packaging aggregated sources: the " + AGGREGATE_GOAL + " goal requires \"" + Type.POM
+                    + "\" packaging, but this project uses \"" + packaging + "\" packaging.");
         }
     }
 }

src/test/java/org/apache/maven/plugins/source/AggregatorSourceJarMojoTest.java

@@ -0,0 +1,83 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.maven.plugins.source;
+
+import org.apache.maven.api.Project;
+import org.apache.maven.api.Type;
+import org.apache.maven.api.plugin.Log;
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.RETURNS_DEEP_STUBS;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+/**
+ * Tests that the aggregate goal reports why it did nothing when the project
+ * does not use "pom" packaging, instead of failing silently.
+ */
+class AggregatorSourceJarMojoTest {
+
+    private AggregatorSourceJarMojo mojoForPackaging(String packaging, Log log) {
+        Project project = mock(Project.class, RETURNS_DEEP_STUBS);
+        when(project.getPackaging().type().id()).thenReturn(packaging);
+
+        AggregatorSourceJarMojo mojo = new AggregatorSourceJarMojo();
+        mojo.project = project;
+        mojo.log = log;
+        return mojo;
+    }
+
+    @Test
+    void warnsWhenPackagingIsNotPom() {
+        Log log = mock(Log.class);
+        AggregatorSourceJarMojo mojo = mojoForPackaging("jar", log);
+
+        mojo.doExecute();
+
+        ArgumentCaptor<CharSequence> message = ArgumentCaptor.forClass(CharSequence.class);
+        verify(log).warn(message.capture());
+
+        String warning = message.getValue().toString();
+        assertTrue(warning.contains("jar"), "warning should name the actual packaging, was: " + warning);
+        assertTrue(warning.contains(Type.POM), "warning should name the required packaging, was: " + warning);
+        assertTrue(
+                warning.contains(AggregatorSourceJarMojo.AGGREGATE_GOAL),
+                "warning should name the goal, was: " + warning);
+    }
+
+    @Test
+    void doesNotWarnWhenPackagingIsPom() {
+        Log log = mock(Log.class);
+        AggregatorSourceJarMojo mojo = mojoForPackaging(Type.POM, log);
+
+        // reactorProjects is left null: packageSources would throw before any warning
+        // is emitted, which is enough to show the non-pom branch was not taken.
+        try {
+            mojo.doExecute();
+        } catch (RuntimeException expected) {
+            // not the subject of this test
+        }
+
+        verify(log, never()).warn(org.mockito.ArgumentMatchers.<CharSequence>any());
+    }
+}