Warn when aggregate runs on non-POM packaging

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

@@ -36,8 +36,12 @@ public class AggregatorSourceJarMojo extends SourceJarMojo {
      */
     @Override
     protected void doExecute() throws MojoException {
-        if (Type.POM.equals(getProject().getPackaging().type().id())) {
+        String type = getProject().getPackaging().type().id();
+        if (Type.POM.equals(type)) {
             packageSources(reactorProjects);
+        } else {
+            getLog().warn("NOT aggregating sources as this goal requires a project with [" + Type.POM
+                    + "] packaging. Current project [" + getProject().getId() + "] has a [" + type + "] packaging.");
         }
     }
 }

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

@@ -0,0 +1,103 @@
+/*
+ * 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 java.util.Collections;
+import java.util.List;
+
+import org.apache.maven.api.Packaging;
+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.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.verifyNoInteractions;
+import static org.mockito.Mockito.when;
+
+class AggregatorSourceJarMojoTest {
+
+    /**
+     * Records the projects handed to {@code packageSources} instead of building an archive, so the test can tell
+     * whether the goal did its work or skipped.
+     */
+    private static class RecordingAggregatorSourceJarMojo extends AggregatorSourceJarMojo {
+        private List<Project> packagedProjects;
+
+        @Override
+        protected void packageSources(List<Project> theProjects) {
+            packagedProjects = theProjects;
+        }
+    }
+
+    @Test
+    void testPomPackagingPackagesTheReactorProjects() {
+        Log log = mock(Log.class);
+        RecordingAggregatorSourceJarMojo mojo = mojoWithPackaging(Type.POM, log);
+        List<Project> reactorProjects = Collections.singletonList(mock(Project.class));
+        mojo.reactorProjects = reactorProjects;
+
+        mojo.doExecute();
+
+        assertEquals(reactorProjects, mojo.packagedProjects);
+        verifyNoInteractions(log);
+    }
+
+    /**
+     * The goal cannot aggregate anything for non-POM packaging, but skipping without a word leaves the user with no
+     * source archive and no reason why.
+     */
+    @Test
+    void testNonPomPackagingWarnsInsteadOfSkippingSilently() {
+        Log log = mock(Log.class);
+        RecordingAggregatorSourceJarMojo mojo = mojoWithPackaging("jar", log);
+
+        mojo.doExecute();
+
+        assertNull(mojo.packagedProjects, "Sources should not be packaged for non-POM packaging");
+
+        ArgumentCaptor<CharSequence> warning = ArgumentCaptor.forClass(CharSequence.class);
+        verify(log).warn(warning.capture());
+        String message = warning.getValue().toString();
+        assertTrue(message.contains("jar"), "Warning should name the actual packaging: " + message);
+        assertTrue(message.contains("aggregate:1.0:pom"), "Warning should name the project: " + message);
+    }
+
+    private static RecordingAggregatorSourceJarMojo mojoWithPackaging(String type, Log log) {
+        Type packagingType = mock(Type.class);
+        when(packagingType.id()).thenReturn(type);
+
+        Packaging packaging = mock(Packaging.class);
+        when(packaging.type()).thenReturn(packagingType);
+
+        Project project = mock(Project.class);
+        when(project.getPackaging()).thenReturn(packaging);
+        when(project.getId()).thenReturn("aggregate:1.0:pom");
+
+        RecordingAggregatorSourceJarMojo mojo = new RecordingAggregatorSourceJarMojo();
+        mojo.project = project;
+        mojo.log = log;
+        return mojo;
+    }
+}