Add an PathType.UNRESOLVED value for allowing plugins to log warnin…

…g to users when a dependency cannot be placed on any path.

api/maven-api-core/src/main/java/org/apache/maven/api/PathType.java

@@ -37,6 +37,35 @@
  */
 @Experimental
 public interface PathType {
+    /**
+     * The type for all paths that could not be placed in any of the types requested by a caller.
+     * This type can appear in the return value of a call to
+     * {@link Session#resolveDependencies resolveDependencies(...)} when at least one dependency
+     * cannot be associated to any type specified in the {@code desiredTypes} argument.
+     * Plugins can choose to report a warning to users when unresolved paths exist.
+     */
+    PathType UNRESOLVED = new PathType() {
+        @Override
+        public String name() {
+            return "UNRESOLVED";
+        }
+
+        @Override
+        public String id() {
+            return "UNRESOLVED";
+        }
+
+        @Override
+        public Optional<String> option() {
+            return Optional.empty();
+        }
+
+        @Override
+        public String option(Iterable<? extends Path> paths) {
+            return "";
+        }
+    };
+
     /**
      * Returns the unique name of this path type, including the module to patch if any.
      * For example, if this type is {@link JavaPathType#MODULES}, then this method returns {@code "MODULES"}.

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

@@ -266,7 +266,7 @@ void addDependency(Node node, Dependency dep, Predicate<PathType> filter, Path p
                 return; // Dependency added, we are done.
             }
         }
-        cache.selectPathType(pathTypes, filter, path).ifPresent((type) -> addPathElement(type, path));
+        addPathElement(cache.selectPathType(pathTypes, filter, path).orElse(PathType.UNRESOLVED), path);
     }
 
     /**

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

@@ -90,7 +90,7 @@ private PathType getPathType(Path path) throws IOException {
 
     /**
      * Selects the type of path where to place the given dependency.
-     * This method returns one of the values specified in the given array.
+     * This method returns one of the values specified in the given collection.
      * This method does not handle the patch-module paths, because the patches
      * depend on which modules have been previously added on the module-paths.
      *

maven-core/src/test/java/org/apache/maven/internal/impl/TestApi.java

@@ -185,9 +185,12 @@ void testResolveArtifactCoordinateDependencies() {
                 coord, PathScope.TEST_COMPILE, Arrays.asList(JavaPathType.CLASSES, JavaPathType.MODULES));
         List<Path> classes = dispatched.get(JavaPathType.CLASSES);
         List<Path> modules = dispatched.get(JavaPathType.MODULES);
-        assertEquals(2, dispatched.size());
-        assertEquals(8, classes.size()); // "pluxus.pom" and "junit.jar" are excluded.
+        List<Path> unresolved = dispatched.get(PathType.UNRESOLVED);
+        assertEquals(3, dispatched.size());
+        assertEquals(1, unresolved.size());
+        assertEquals(8, classes.size()); // "plexus.pom" and "junit.jar" are excluded.
         assertEquals(1, modules.size());
+        assertEquals("plexus-1.0.11.pom", unresolved.get(0).getFileName().toString());
         assertEquals("test-extension-1.jar", classes.get(0).getFileName().toString());
         assertEquals("junit-4.13.1.jar", modules.get(0).getFileName().toString());
         assertTrue(paths.containsAll(classes));
@@ -197,10 +200,13 @@ void testResolveArtifactCoordinateDependencies() {
         dispatched = session.resolveDependencies(coord, PathScope.TEST_COMPILE, Arrays.asList(JavaPathType.CLASSES));
         classes = dispatched.get(JavaPathType.CLASSES);
         modules = dispatched.get(JavaPathType.MODULES);
-        assertEquals(1, dispatched.size());
+        unresolved = dispatched.get(PathType.UNRESOLVED);
+        assertEquals(2, dispatched.size());
+        assertEquals(1, unresolved.size());
         assertEquals(9, classes.size());
         assertNull(modules);
         assertTrue(paths.containsAll(classes));
+        assertEquals("plexus-1.0.11.pom", unresolved.get(0).getFileName().toString());
     }
 
     @Test