Removing test-scope dependencies before building dependency tree

Fixes mojohaus#185

src/main/java/org/codehaus/mojo/flatten/FlattenMojo.java

@@ -19,6 +19,7 @@
  * under the License.
  */
 
+import java.util.LinkedHashSet;
 import java.util.Queue;
 import org.apache.maven.artifact.Artifact;
 import org.apache.maven.artifact.factory.ArtifactFactory;
@@ -1070,11 +1071,11 @@ private void createFlattenedDependenciesDirect( List<Dependency> projectDependen
      * The collected dependencies are stored in order, so that the leaf dependencies are prioritized in front of direct dependencies.
      * In addition, every non-leaf dependencies will exclude its own direct dependency, since all transitive dependencies
      * will be collected.
-	 *
+     *
      * Transitive dependencies are all going to be collected and become a direct dependency. Maven should already resolve
      * versions properly because now the transitive dependencies are closer to the artifact. However, when this artifact is
      * being consumed, Maven Enforcer Convergence rule will fail because there may be multiple versions for the same transitive dependency.
-	 *
+	   *
      * Typically, exclusion can be done by using the wildcard. However, a known Maven issue prevents convergence enforcer from
      * working properly w/ wildcard exclusions. Thus, this will exclude each dependencies explicitly rather than using the wildcard.
      *
@@ -1091,11 +1092,31 @@ private void createFlattenedDependenciesAll( List<Dependency> projectDependencie
 
         final Artifact projectArtifact = this.project.getArtifact();
 
-        // The test-scope project dependencies will hinder transitive dependencies by marking them
-        // as 'omitted for duplicate.'
-        this.project.getDependencies().removeIf(dependency -> "test".equals(dependency.getScope()));
+        // The test-scope project dependencies may hinder transitive dependencies by marking them as 'omitted for duplicate' when
+        // building dependency tree. This is a problem when the transitive dependency is actually needed by another non-test dependency
+        // of the project. To avoid this interference of test-scope project dependencies, this plugin builds a dependency tree
+        // of the project without direct, test-scope dependencies. This is safe because these dependencies do not appear in
+        // library users' class path in any case.
+        final Set<String> testScopeProjectDependencyKeys = new HashSet<>();
+        for (Dependency projectDependency : projectDependencies)
+        {
+            if ("test".equals(projectDependency.getScope()))
+            {
+                testScopeProjectDependencyKeys.add(projectDependency.getManagementKey());
+            }
+        }
+        // LinkedHashSet preserves the order.
+        final Set<Artifact> dependencyArtifactsWithoutTest = new LinkedHashSet<>(this.project.getDependencyArtifacts());
+        dependencyArtifactsWithoutTest.removeIf(artifact -> {
+            // The same logic as org.apache.maven.model.Dependency.getManagementKey()
+            String managementKey = artifact.getGroupId() + ":" + artifact.getArtifactId() + ":" + artifact.getType()
+                + (artifact.getClassifier() != null ? ":" + artifact.getClassifier() : "");
+            return testScopeProjectDependencyKeys.contains(managementKey);
+        });
+        final MavenProject projectWithoutTestScopeDeps = this.project.clone();
+        projectWithoutTestScopeDeps.setDependencyArtifacts(dependencyArtifactsWithoutTest);
 
-        final DependencyNode dependencyNode = this.dependencyTreeBuilder.buildDependencyTree(this.project,
+        final DependencyNode dependencyNode = this.dependencyTreeBuilder.buildDependencyTree(projectWithoutTestScopeDeps,
                 this.localRepository, null);
 
         dependencyNode.accept(new DependencyNodeVisitor()