Support KClass.objectInstance for objects and companion objects

compiler/testData/codegen/boxWithStdlib/reflection/classes/objectInstance.kt

@@ -0,0 +1,29 @@
+import kotlin.test.assertEquals
+
+object Obj {
+    fun foo() = 1
+}
+
+class A {
+    companion object {
+        fun foo() = 2
+    }
+}
+
+class B {
+    companion object Factory {
+        fun foo() = 3
+    }
+}
+
+class C
+
+fun box(): String {
+    assertEquals(1, Obj::class.objectInstance!!.foo())
+    assertEquals(2, A.Companion::class.objectInstance!!.foo())
+    assertEquals(3, B.Factory::class.objectInstance!!.foo())
+
+    assertEquals(null, C::class.objectInstance)
+
+    return "OK"
+}

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java

@@ -2994,6 +2994,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
                 doTestWithStdlib(fileName);
             }
 
+            @TestMetadata("objectInstance.kt")
+            public void testObjectInstance() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/classes/objectInstance.kt");
+                doTestWithStdlib(fileName);
+            }
+
             @TestMetadata("qualifiedName.kt")
             public void testQualifiedName() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/classes/qualifiedName.kt");

core/builtins/src/kotlin/reflect/KClass.kt

@@ -24,7 +24,7 @@ package kotlin.reflect
  *
  * @param T the type of the class.
  */
-public interface KClass<T> : KDeclarationContainer, KAnnotatedElement {
+public interface KClass<T : Any> : KDeclarationContainer, KAnnotatedElement {
     /**
      * The simple name of the class as it was declared in the source code,
      * or `null` if the class has no name (if, for example, it is an anonymous object literal).
@@ -48,4 +48,9 @@ public interface KClass<T> : KDeclarationContainer, KAnnotatedElement {
      * All constructors declared in this class.
      */
     public val constructors: Collection<KFunction<T>>
+
+    /**
+     * The instance of the object declaration, or `null` if this class is not an object declaration.
+     */
+    public val objectInstance: T?
 }

core/reflection.jvm/src/kotlin/reflect/jvm/internal/KClassImpl.kt

@@ -19,6 +19,7 @@ package kotlin.reflect.jvm.internal
 import org.jetbrains.kotlin.descriptors.ClassKind
 import org.jetbrains.kotlin.descriptors.ConstructorDescriptor
 import org.jetbrains.kotlin.descriptors.annotations.Annotated
+import org.jetbrains.kotlin.load.java.JvmAbi
 import org.jetbrains.kotlin.name.ClassId
 import org.jetbrains.kotlin.resolve.scopes.ChainedScope
 import org.jetbrains.kotlin.resolve.scopes.JetScope
@@ -27,7 +28,7 @@ import kotlin.reflect.KClass
 import kotlin.reflect.KFunction
 import kotlin.reflect.KotlinReflectionInternalError
 
-class KClassImpl<T>(override val jClass: Class<T>) : KCallableContainerImpl(), KClass<T>, KAnnotatedElementImpl {
+class KClassImpl<T : Any>(override val jClass: Class<T>) : KCallableContainerImpl(), KClass<T>, KAnnotatedElementImpl {
     val descriptor by ReflectProperties.lazySoft {
         val classId = classId
 
@@ -92,6 +93,20 @@ class KClassImpl<T>(override val jClass: Class<T>) : KCallableContainerImpl(), K
             KFunctionImpl(this, it) as KFunction<T>
         }
 
+    @suppress("UNCHECKED_CAST")
+    override val objectInstance: T? by ReflectProperties.lazy {
+        val descriptor = descriptor
+        if (descriptor.kind != ClassKind.OBJECT) return@lazy null
+
+        val field = if (descriptor.isCompanionObject) {
+            jClass.enclosingClass.getDeclaredField(descriptor.name.asString())
+        }
+        else {
+            jClass.getDeclaredField(JvmAbi.INSTANCE_FIELD)
+        }
+        field.get(null) as T
+    }
+
     override fun equals(other: Any?): Boolean =
             other is KClassImpl<*> && jClass == other.jClass
 

core/runtime.jvm/src/kotlin/jvm/internal/ClassReference.kt

@@ -36,5 +36,8 @@ public class ClassReference(override val jClass: Class<*>) : KClass<Any>, Declar
     override val annotations: List<Annotation>
         get() = error()
 
+    override val objectInstance: Any?
+        get() = error()
+
     private fun error(): Nothing = throw KotlinReflectionNotSupportedError()
 }