Breaking change: make publish {...} require <T: Any>

Judging by ReactiveX/RxJava#4644, the
requirement 2.13 from the reactive spec
(https://github.com/reactive-streams/reactive-streams-jvm#2.13)
does mean that null values can't be passed to `onNext`. Thus, it
is incorrect for `publish {...}` to accept nullable values.

Author: dkhalanskyjb

reactive/kotlinx-coroutines-jdk9/src/Publish.kt

@@ -33,7 +33,7 @@ import org.reactivestreams.FlowAdapters
  * @throws IllegalArgumentException if the provided [context] contains a [Job] instance.
  */
 @ExperimentalCoroutinesApi
-public fun <T> flowPublish(
+public fun <T : Any> flowPublish(
     context: CoroutineContext = EmptyCoroutineContext,
     @BuilderInference block: suspend ProducerScope<T>.() -> Unit
 ): Flow.Publisher<T> = FlowAdapters.toFlowPublisher(publish(context, block))

reactive/kotlinx-coroutines-reactive/src/Convert.kt

@@ -4,6 +4,7 @@
 
 package kotlinx.coroutines.reactive
 
+import kotlinx.coroutines.*
 import kotlinx.coroutines.channels.*
 import org.reactivestreams.*
 import kotlin.coroutines.*
@@ -18,7 +19,10 @@ import kotlin.coroutines.*
 @Deprecated(message = "Deprecated in the favour of consumeAsFlow()",
     level = DeprecationLevel.WARNING, // Error in 1.4
     replaceWith = ReplaceWith("this.consumeAsFlow().asPublisher()"))
-public fun <T> ReceiveChannel<T>.asPublisher(context: CoroutineContext = EmptyCoroutineContext): Publisher<T> = publish(context) {
-    for (t in this@asPublisher)
-        send(t)
-}
+public fun <T> ReceiveChannel<T>.asPublisher(context: CoroutineContext = EmptyCoroutineContext): Publisher<T> =
+    // we call the deprecated version here because the non-deprecated one requires <T: Any> now
+    @Suppress("DEPRECATION_ERROR")
+    GlobalScope.publish(context) {
+        for (t in this@asPublisher)
+            send(t)
+    }

reactive/kotlinx-coroutines-reactive/src/Publish.kt

@@ -37,7 +37,7 @@ import kotlin.internal.*
  * @throws IllegalArgumentException if the provided [context] contains a [Job] instance.
  */
 @ExperimentalCoroutinesApi
-public fun <T> publish(
+public fun <T : Any> publish(
     context: CoroutineContext = EmptyCoroutineContext,
     @BuilderInference block: suspend ProducerScope<T>.() -> Unit
 ): Publisher<T> {