Fixed regularized 'when' inference to build train

See https://youtrack.jetbrains.com/issue/KT-36776

Author: elizarov

kotlinx-coroutines-core/common/src/channels/ConflatedBroadcastChannel.kt

@@ -91,7 +91,7 @@ public class ConflatedBroadcastChannel<E>() : BroadcastChannel<E> {
      */
     public val valueOrNull: E? get() = when (val state = _state.value) {
         is Closed -> null
-        is State<*> -> UNDEFINED.unbox(state.value)
+        is State<*> -> UNDEFINED.unbox<E?>(state.value)
         else -> error("Invalid state $state")
     }