override fun visitFileTree(root: File, patterns: PatternSet, fileTree: FileTreeInternal) =
        unsupportedFileTree(fileTree)

    override fun visitFileTreeBackedByFile(file: File, fileTree: FileTreeInternal, sourceTree: FileSystemMirroringFileTree) =
        unsupportedFileTree(fileTree)

    private
    fun unsupportedFileTree(fileTree: FileTreeInternal): Nothing =
        throw UnsupportedOperationException(

            }

            private void addTreeContents(FileTreeInternal fileTree) {
                // TODO - add some convenient way to visit the files of the tree without collecting them into a set
                files.addAll(fileTree.getFiles());
            }

            @Override
            public void visitFileTree(File root, PatternSet patterns, FileTreeInternal fileTree) {
                addTreeContents(fileTree);

        return new DefaultConfigurableFileTree(fileResolver, listener, patternSetFactory, taskDependencyFactory, directoryFileTreeFactory);
    }

    @Override
    public FileTreeInternal treeOf(List<? extends FileTreeInternal> fileTrees) {
        if (fileTrees.isEmpty()) {
            return FileCollectionFactory.emptyTree();
        } else if (fileTrees.size() == 1) {
            return fileTrees.get(0);

            void visitFileTree(File root, PatternSet patterns, FileTreeInternal fileTree) {
                assert root == classesDir
                assert patterns.getIncludes() == includes
                assert patterns.getExcludes() == excludes
            }

            @Override
            void visitFileTreeBackedByFile(File file, FileTreeInternal fileTree, FileSystemMirroringFileTree sourceTree) {

import org.gradle.api.internal.TaskInternal;
import org.gradle.api.internal.file.FileCollectionInternal;
import org.gradle.api.internal.file.FileCollectionStructureVisitor;
import org.gradle.api.internal.file.FileTreeInternal;
import org.gradle.api.internal.file.collections.FileSystemMirroringFileTree;
import org.gradle.api.problems.Severity;
import org.gradle.api.problems.internal.GradleCoreProblemGroup;
import org.gradle.api.specs.Spec;

    }

    @Override
    public FileTreeInternal zipTree(Object zipPath) {
        Provider<File> fileProvider = asFileProvider(zipPath);
        return new FileTreeAdapter(new ZipFileTree(fileProvider, fileSystem, directoryFileTreeFactory, fileHasher, decompressionCoordinator, temporaryFileProvider), taskDependencyFactory, patternSetFactory);
    }

    @Override
    public FileTreeInternal zipTreeNoLocking(Object zipPath) {

            Assert.fail()
        }


        @Override
        void visitFileTree(File root, PatternSet patterns, FileTreeInternal fileTree) {
            Assert.fail()
        }

        @Override
        void visitFileTreeBackedByFile(File file, FileTreeInternal fileTree, FileSystemMirroringFileTree sourceTree) {
            Assert.fail()
        }
    }

        DefaultGroovyJavaJointCompileSpec spec = new DefaultGroovyJavaJointCompileSpecFactory(compileOptions, getToolchain()).create();
        assert spec != null;

        FileTreeInternal stableSourcesAsFileTree = (FileTreeInternal) getStableSources().getAsFileTree();
        List<File> sourceRoots = CompilationSourceDirs.inferSourceRoots(stableSourcesAsFileTree);

        spec.setSourcesRoots(sourceRoots);

            }

            @Override
            public void visitFileTree(File root, PatternSet patterns, FileTreeInternal fileTree) {
            }

            @Override
            public void visitFileTreeBackedByFile(File file, FileTreeInternal fileTree, FileSystemMirroringFileTree sourceTree) {
            }
        });

        setFrom(Cast.castNullable(FileCollection.class, object));

    }

    def "visits children when visitor requests contents"() {
        def child1 = Mock(FileCollectionInternal)
        def child2 = Mock(FileTreeInternal)
        def collection = new TestCollection() {
            @Override
            protected void visitChildren(Consumer<FileCollectionInternal> visitor) {
                visitor.accept(child1)