Added deprecated (with error level) plus operators on Job and CoroutineDispatcher to catch potential problem with their incorrect use at compile time

Author: elizarov

kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/CoroutineDispatcher.kt

@@ -36,6 +36,13 @@ public abstract class CoroutineDispatcher :
 
     override fun <T> interceptContinuation(continuation: Continuation<T>): Continuation<T> =
             DispatchedContinuation<T>(this, continuation)
+
+    @Suppress("DeprecatedCallableAddReplaceWith")
+    @Deprecated(message = "Operator '+' on two CoroutineDispatcher objects is meaningless. " +
+            "CoroutineDispatcher is a coroutine context element and `+` is a set-sum operator for coroutine contexts. " +
+            "The dispatcher to the right of `+` just replaces the dispacher the left of `+`.",
+            level = DeprecationLevel.ERROR)
+    public operator fun plus(other: CoroutineDispatcher) = other
 }
 
 internal class DispatchedContinuation<T>(

kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/Job.kt

@@ -68,6 +68,13 @@ public interface Job : CoroutineContext.Element {
      */
     public fun cancel(reason: Throwable? = null): Boolean
 
+    @Suppress("DeprecatedCallableAddReplaceWith")
+    @Deprecated(message = "Operator '+' on two Job objects is meaningless. " +
+                    "Job is a coroutine context element and `+` is a set-sum operator for coroutine contexts. " +
+                    "The job to the right of `+` just replaces the job the left of `+`.",
+            level = DeprecationLevel.ERROR)
+    public operator fun plus(other: Job) = other
+
     /**
      * Registration object for [onCompletion]. It can be used to [unregister] if needed.
      * There is no need to unregister after completion.