KT-51082 introduce Class<E : Enum<E>>.declaringJavaClass as a future replacement for synthetic declaringClass

libraries/stdlib/jvm/runtime/kotlin/jvm/JvmClassMapping.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2022 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 @file:JvmName("JvmClassMappingKt")
@@ -7,6 +7,7 @@
 
 package kotlin.jvm
 
+import kotlin.internal.InlineOnly
 import kotlin.jvm.internal.ClassBasedDeclarationContainer
 import kotlin.jvm.internal.Reflection
 import kotlin.reflect.KClass
@@ -105,3 +106,13 @@ public fun <reified T : Any> Array<*>.isArrayOf(): Boolean =
  */
 public val <T : Annotation> T.annotationClass: KClass<out T>
     get() = (this as java.lang.annotation.Annotation).annotationType().kotlin as KClass<out T>
+
+/**
+ * Returns a Java [Class] instance of the enum the given constant belongs to.
+ *
+ * @see java.lang.Enum.getDeclaringClass
+ */
+@SinceKotlin("1.7")
+@InlineOnly
+public inline val <E : Enum<E>> E.declaringJavaClass: Class<E>
+    get() = (this as java.lang.Enum<E>).declaringClass

libraries/stdlib/jvm/test/enums/EnumDeclaringJavaClassTest.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2010-2022 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package test.enums
+
+import java.util.*
+import java.util.concurrent.TimeUnit
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.time.DurationUnit
+
+class EnumDeclaringJavaClassTest {
+
+    private enum class TestEnum {
+        E
+    }
+
+    @Test
+    fun testDeclaringClass() {
+        // This file
+        assertEquals(TestEnum::class.java, TestEnum.E.declaringJavaClass)
+        // From Java
+        assertEquals(TimeUnit::class.java, TimeUnit.MILLISECONDS.declaringJavaClass)
+        // From Kotlin
+        assertEquals(DurationUnit::class.java, DurationUnit.MILLISECONDS.declaringJavaClass)
+    }
+
+    private inline fun <reified E : Enum<E>> E.declaring() = declaringJavaClass
+
+    @Test
+    fun testReified() {
+        // This file
+        assertEquals(TestEnum::class.java, TestEnum.E.declaring())
+        // From Java
+        assertEquals(TimeUnit::class.java, TimeUnit.MILLISECONDS.declaring())
+    }
+
+    @Test
+    fun testEnumSet() {
+        val set = EnumSet.noneOf(TestEnum.E.declaringJavaClass)
+        set.addAll(TestEnum.E.declaringJavaClass.enumConstants.toList())
+        assertEquals(EnumSet.of(TestEnum.E), set)
+    }
+}