Consumer POM of multi-module project should exclude <build> and <dependencies> elements.

Author: desruisseaux

impl/maven-core/src/main/java/org/apache/maven/internal/transformation/impl/ConsumerPomArtifactTransformer.java

@@ -96,7 +96,7 @@ public void injectTransformedArtifacts(RepositorySystemSession session, MavenPro
         }
     }
 
-    TransformedArtifact createConsumerPomArtifact(
+    private TransformedArtifact createConsumerPomArtifact(
             MavenProject project, Path consumer, RepositorySystemSession session) {
         Path actual = project.getFile().toPath();
         Path parent = project.getBaseDirectory();

impl/maven-core/src/main/java/org/apache/maven/internal/transformation/impl/DefaultConsumerPomBuilder.java

@@ -32,6 +32,7 @@
 import org.apache.maven.api.Node;
 import org.apache.maven.api.PathScope;
 import org.apache.maven.api.SessionData;
+import org.apache.maven.api.SourceRoot;
 import org.apache.maven.api.feature.Features;
 import org.apache.maven.api.model.Dependency;
 import org.apache.maven.api.model.DistributionManagement;
@@ -342,7 +343,7 @@ static Model transformNonPom(Model model, MavenProject project) {
         return model;
     }
 
-    static Model transformBom(Model model, MavenProject project) {
+    private static Model transformBom(Model model, MavenProject project) {
         boolean preserveModelVersion = model.isPreserveModelVersion();
 
         Model.Builder builder = prune(
@@ -372,7 +373,13 @@ static Model transformPom(Model model, MavenProject project) {
         if (model.getParent() != null) {
             model = model.withParent(model.getParent().withRelativePath(null));
         }
-
+        if (isModular(project)) {
+            // Build information are not really needed by consumers. Removed because the <source> elements
+            // are not compatible with Maven 4.0.0 model, and the remaining is not valid without them.
+            model = model.withBuild(null);
+            // Dependencies are dispatched by maven-jar-plugin in the POM generated for each module.
+            model = model.withDependencies(null);
+        }
         if (!preserveModelVersion) {
             model = model.withPreserveModelVersion(false);
             String modelVersion = new MavenModelVersion().getModelVersion(model);
@@ -381,7 +388,22 @@ static Model transformPom(Model model, MavenProject project) {
         return model;
     }
 
-    static void warnNotDowngraded(MavenProject project) {
+    /**
+     * Whether the given project is modular. This method returns {@code true} it at least one enabled
+     * {@code <source>} element declares a Java modules. While modular and non-modular sources should
+     * not be mixed, this code is tolerant to such mixes because non-modular source elements may have
+     * been incorrectly generated by non module-aware codes, in which case they should be ignored.
+     */
+    private static boolean isModular(MavenProject project) {
+        for (SourceRoot source : project.getSourceRoots()) {
+            if (source.enabled() && source.module().isPresent()) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private static void warnNotDowngraded(MavenProject project) {
         LOGGER.warn("The consumer POM for " + project.getId() + " cannot be downgraded to 4.0.0. "
                 + "If you intent your build to be consumed with Maven 3 projects, you need to remove "
                 + "the features that request a newer model version.  If you're fine with having the "