Introduce CompletableDeferred.asDeferred that prevents downcasting (#4410)

Fixes #4408

Author: dkhalanskyjb

kotlinx-coroutines-core/api/kotlinx-coroutines-core.api

@@ -124,6 +124,7 @@ public final class kotlinx/coroutines/CompletableDeferredKt {
 	public static final fun CompletableDeferred (Ljava/lang/Object;)Lkotlinx/coroutines/CompletableDeferred;
 	public static final fun CompletableDeferred (Lkotlinx/coroutines/Job;)Lkotlinx/coroutines/CompletableDeferred;
 	public static synthetic fun CompletableDeferred$default (Lkotlinx/coroutines/Job;ILjava/lang/Object;)Lkotlinx/coroutines/CompletableDeferred;
+	public static final fun asDeferred (Lkotlinx/coroutines/CompletableDeferred;)Lkotlinx/coroutines/Deferred;
 	public static final fun completeWith (Lkotlinx/coroutines/CompletableDeferred;Ljava/lang/Object;)Z
 }
 

kotlinx-coroutines-core/api/kotlinx-coroutines-core.klib.api

@@ -907,6 +907,7 @@ final fun <#A: kotlin/Any?> (kotlinx.coroutines.flow/StateFlow<#A>).kotlinx.coro
 final fun <#A: kotlin/Any?> (kotlinx.coroutines.flow/StateFlow<#A>).kotlinx.coroutines.flow/onSubscription(kotlin.coroutines/SuspendFunction1<kotlinx.coroutines.flow/FlowCollector<#A>, kotlin/Unit>): kotlinx.coroutines.flow/StateFlow<#A> // kotlinx.coroutines.flow/onSubscription|[email protected]<0:0>(kotlin.coroutines.SuspendFunction1<kotlinx.coroutines.flow.FlowCollector<0:0>,kotlin.Unit>){0§<kotlin.Any?>}[0]
 final fun <#A: kotlin/Any?> (kotlinx.coroutines.selects/SelectBuilder<#A>).kotlinx.coroutines.selects/onTimeout(kotlin.time/Duration, kotlin.coroutines/SuspendFunction0<#A>) // kotlinx.coroutines.selects/onTimeout|[email protected]<0:0>(kotlin.time.Duration;kotlin.coroutines.SuspendFunction0<0:0>){0§<kotlin.Any?>}[0]
 final fun <#A: kotlin/Any?> (kotlinx.coroutines.selects/SelectBuilder<#A>).kotlinx.coroutines.selects/onTimeout(kotlin/Long, kotlin.coroutines/SuspendFunction0<#A>) // kotlinx.coroutines.selects/onTimeout|[email protected]<0:0>(kotlin.Long;kotlin.coroutines.SuspendFunction0<0:0>){0§<kotlin.Any?>}[0]
+final fun <#A: kotlin/Any?> (kotlinx.coroutines/CompletableDeferred<#A>).kotlinx.coroutines/asDeferred(): kotlinx.coroutines/Deferred<#A> // kotlinx.coroutines/asDeferred|[email protected]<0:0>(){0§<kotlin.Any?>}[0]
 final fun <#A: kotlin/Any?> (kotlinx.coroutines/CompletableDeferred<#A>).kotlinx.coroutines/completeWith(kotlin/Result<#A>): kotlin/Boolean // kotlinx.coroutines/completeWith|[email protected]<0:0>(kotlin.Result<0:0>){0§<kotlin.Any?>}[0]
 final fun <#A: kotlin/Any?> (kotlinx.coroutines/CoroutineScope).kotlinx.coroutines.channels/broadcast(kotlin.coroutines/CoroutineContext = ..., kotlin/Int = ..., kotlinx.coroutines/CoroutineStart = ..., kotlin/Function1<kotlin/Throwable?, kotlin/Unit>? = ..., kotlin.coroutines/SuspendFunction1<kotlinx.coroutines.channels/ProducerScope<#A>, kotlin/Unit>): kotlinx.coroutines.channels/BroadcastChannel<#A> // kotlinx.coroutines.channels/broadcast|[email protected](kotlin.coroutines.CoroutineContext;kotlin.Int;kotlinx.coroutines.CoroutineStart;kotlin.Function1<kotlin.Throwable?,kotlin.Unit>?;kotlin.coroutines.SuspendFunction1<kotlinx.coroutines.channels.ProducerScope<0:0>,kotlin.Unit>){0§<kotlin.Any?>}[0]
 final fun <#A: kotlin/Any?> (kotlinx.coroutines/CoroutineScope).kotlinx.coroutines.channels/produce(kotlin.coroutines/CoroutineContext = ..., kotlin/Int = ..., kotlin.coroutines/SuspendFunction1<kotlinx.coroutines.channels/ProducerScope<#A>, kotlin/Unit>): kotlinx.coroutines.channels/ReceiveChannel<#A> // kotlinx.coroutines.channels/produce|[email protected](kotlin.coroutines.CoroutineContext;kotlin.Int;kotlin.coroutines.SuspendFunction1<kotlinx.coroutines.channels.ProducerScope<0:0>,kotlin.Unit>){0§<kotlin.Any?>}[0]

kotlinx-coroutines-core/common/src/CompletableDeferred.kt

@@ -69,6 +69,37 @@ public fun <T> CompletableDeferred(parent: Job? = null): CompletableDeferred<T>
 @Suppress("FunctionName")
 public fun <T> CompletableDeferred(value: T): CompletableDeferred<T> = CompletableDeferredImpl<T>(null).apply { complete(value) }
 
+/**
+ * Creates a view of this [CompletableDeferred] as a [Deferred], which prevents downcasting to a completable version.
+ *
+ * ```
+ * class MyClass(val scope: CoroutineScope) {
+ *     // can be completed
+ *     private val actualDeferred: CompletableDeferred<String> = CompletableDeferred()
+ *
+ *     // can not be completed from outside
+ *     public val operationCompleted: Deferred<String> = actualDeferred.asDeferred()
+ *
+ *     fun startOperation() = scope.launch {
+ *         // do some work
+ *         delay(2.seconds)
+ *         actualDeferred.complete("Done")
+ *     }
+ * }
+ *
+ * // (myClass.operationCompleted as CompletableDeferred<*>) will fail
+ * ```
+ */
+@ExperimentalCoroutinesApi
+public fun <T> CompletableDeferred<T>.asDeferred(): Deferred<T> = ReadonlyDeferred(this)
+
+@OptIn(InternalForInheritanceCoroutinesApi::class)
+private class ReadonlyDeferred<T>(
+    val deferred: CompletableDeferred<T>,
+) : Deferred<T> by deferred {
+    override fun toString(): String = "ReadonlyDeferred($deferred)"
+}
+
 /**
  * Concrete implementation of [CompletableDeferred].
  */