Allow Kotlin lambda as deferred path/file notation

Resolves gradle/kotlin-dsl-samples#1077

gradle/dependencies.gradle

@@ -69,6 +69,7 @@ libraries.jsch =                [coordinates: 'com.jcraft:jsch', version: '0.1.5
 libraries.jsr305 =              [coordinates: 'com.google.code.findbugs:jsr305', version: '3.0.2']
 libraries.junit =               [coordinates: 'junit:junit', version: '4.12']
 libraries.junit_platform =      [coordinates: 'org.junit.platform:junit-platform-launcher', version: '1.3.1']
+libraries.kotlin =              [coordinates: 'org.jetbrains.kotlin:kotlin-stdlib', version: '1.3.0-rc-116']
 libraries.kryo =                [coordinates: 'com.esotericsoftware.kryo:kryo', version: '2.24.0']
 libraries.maven3 =              [coordinates: 'org.apache.maven:maven-core', version: '3.0.4']
 libraries.maven3_wagon_file =   [coordinates: 'org.apache.maven.wagon:wagon-file', version: '3.0.0', because: '3.1.0 of wagon-http seems to break Digest authentication']

subprojects/integ-test/src/integTest/groovy/org/gradle/integtests/GradleKotlinDslIntegrationTest.groovy

@@ -165,4 +165,27 @@ task("dumpKotlinBuildScriptModelClassPath") {
         then:
         outputContains("gradle-kotlin-dsl!")
     }
+
+    def 'can use Kotlin lambda as path notation'() {
+        given:
+        buildFile << """
+            task("listFiles") {
+                doLast {
+                    val collection = layout.files(
+                        // top-level lambda
+                        { "foo" },
+                        // nested lambda
+                        { file({ "bar" }) }
+                    )
+                    println(collection.files.map { it.name })
+                }
+            }
+        """
+
+        when:
+        succeeds 'listFiles'
+
+        then:
+        outputContains '[foo, bar]'
+    }
 }

subprojects/model-core/model-core.gradle

@@ -28,6 +28,7 @@ dependencies {
     api project(':baseServices')
     api project(":coreApi")
     api libraries.groovy.coordinates
+    api libraries.kotlin.coordinates
 
     implementation project(':baseServicesGroovy')
     implementation libraries.slf4j_api.coordinates

subprojects/model-core/src/main/java/org/gradle/util/DeferredUtil.java

@@ -26,8 +26,8 @@
 
 public class DeferredUtil {
     /**
-     * Successively unpacks a path that may be deferred by a Callable or Factory
-     * until it's resolved to null or something other than a Callable or Factory.
+     * Successively unpacks a path that may be deferred by a Callable, Factory or Kotlin function
+     * until it's resolved to null or something other than a Callable, Factory or Kotlin function.
      */
     @Nullable
     public static Object unpack(@Nullable Object path) {
@@ -39,6 +39,8 @@ public static Object unpack(@Nullable Object path) {
                 return ((Provider<?>) current).get();
             } else if (current instanceof Factory) {
                 return ((Factory) current).create();
+            } else if (current instanceof kotlin.jvm.functions.Function0) {
+                current = ((kotlin.jvm.functions.Function0) current).invoke();
             } else {
                 return current;
             }
@@ -49,6 +51,7 @@ public static Object unpack(@Nullable Object path) {
     public static boolean isDeferred(Object value) {
         return value instanceof Callable
             || value instanceof Provider
-            || value instanceof Factory;
+            || value instanceof Factory
+            || value instanceof kotlin.jvm.functions.Function0;
     }
 }