Fix resolving DirectoryTee elements of a FileCollection.

subprojects/core-api/src/main/java/org/gradle/api/Project.java

@@ -729,6 +729,8 @@ public interface Project extends Comparable<Project>, ExtensionAware, PluginAwar
      *
      * <li>A {@link org.gradle.api.file.FileCollection}. The contents of the collection are included in the returned collection.</li>
      *
+     * <li>A {@link org.gradle.api.file.FileTree} or {@link org.gradle.api.file.DirectoryTree}. The contents of the tree are included in the returned collection.</li>
+     *
      * <li>A {@link Provider} of any supported type. The provider's value is recursively converted to files. If the provider represents an output of a task, that task is executed if the file collection is used as an input to another task.
      *
      * <li>A {@link java.util.concurrent.Callable} that returns any supported type. The return value of the {@code call()} method is recursively converted to files. A {@code null} return value is treated as an empty collection.</li>

subprojects/file-collections/src/integTest/groovy/org/gradle/api/internal/file/FileCollectionIntegrationTest.groovy

@@ -18,6 +18,7 @@ package org.gradle.api.internal.file
 
 import org.gradle.api.tasks.TasksWithInputsAndOutputs
 import org.gradle.integtests.fixtures.AbstractIntegrationSpec
+import spock.lang.Issue
 import spock.lang.Unroll
 
 class FileCollectionIntegrationTest extends AbstractIntegrationSpec implements TasksWithInputsAndOutputs {
@@ -37,6 +38,26 @@ class FileCollectionIntegrationTest extends AbstractIntegrationSpec implements T
         type << ["Object", "Object[]", "Set", "LinkedHashSet", "List", "LinkedList", "Collection", "FileCollection"]
     }
 
+    @Issue("https://github.com/gradle/gradle/issues/10322")
+    def "can construct file collection from the elements of a source directory set"() {
+        buildFile << """
+            def fileCollection = objects.fileCollection()
+            def sourceDirs = objects.sourceDirectorySet('main', 'main files')
+            sourceDirs.srcDirs("dir1", "dir2")
+            fileCollection.from(sourceDirs.srcDirTrees)
+            println("files = \${fileCollection.files.name.sort()}")
+        """
+
+        given:
+        file("dir1/file1").createFile()
+        file("dir1/file2").createFile()
+        file("dir2/sub/file3").createFile()
+
+        expect:
+        succeeds()
+        outputContains("files = [file1, file2, file3]")
+    }
+
     def "finalized file collection resolves locations and ignores later changes to source paths"() {
         buildFile << """
             def files = objects.fileCollection()

subprojects/file-collections/src/main/java/org/gradle/api/internal/file/collections/UnpackingVisitor.java

@@ -18,6 +18,7 @@
 
 import org.gradle.api.Buildable;
 import org.gradle.api.Task;
+import org.gradle.api.file.DirectoryTree;
 import org.gradle.api.file.FileCollection;
 import org.gradle.api.internal.provider.ProviderInternal;
 import org.gradle.api.internal.tasks.TaskDependencyContainer;
@@ -43,6 +44,10 @@ public void add(@Nullable Object element) {
             context.add(element);
             return;
         }
+        if (element instanceof DirectoryTree) {
+            context.add(element);
+            return;
+        }
         if (element instanceof ProviderInternal) {
             // ProviderInternal is-a TaskDependencyContainer, so check first
             ProviderInternal provider = (ProviderInternal) element;

subprojects/file-collections/src/test/groovy/org/gradle/api/internal/file/collections/UnpackingVisitorTest.groovy

@@ -17,6 +17,7 @@
 package org.gradle.api.internal.file.collections
 
 import org.gradle.api.Task
+import org.gradle.api.file.DirectoryTree
 import org.gradle.api.file.FileCollection
 import org.gradle.api.internal.provider.ProviderInternal
 import org.gradle.api.tasks.TaskOutputs
@@ -202,4 +203,14 @@ class UnpackingVisitorTest extends Specification {
         0 * context._
     }
 
+    def "forwards DirectoryTree"() {
+        def tree = Mock(DirectoryTree)
+
+        when:
+        visitor.add(tree)
+
+        then:
+        1 * context.add(tree)
+        0 * _
+    }
 }