Use Dispatchers.Default on native when no dispatcher is provided (#4075)

Fixes #4074

Author: whyoleg

kotlinx-coroutines-core/common/test/CoroutineScopeTest.kt

@@ -247,6 +247,31 @@ class CoroutineScopeTest : TestBase() {
         finish(7)
     }
 
+    @Test
+    fun testLaunchContainsDefaultDispatcher() = runTest {
+        val scopeWithoutDispatcher = CoroutineScope(coroutineContext.minusKey(ContinuationInterceptor))
+        scopeWithoutDispatcher.launch(Dispatchers.Default) {
+            assertSame(Dispatchers.Default, coroutineContext[ContinuationInterceptor])
+        }.join()
+        scopeWithoutDispatcher.launch {
+            assertSame(Dispatchers.Default, coroutineContext[ContinuationInterceptor])
+        }.join()
+    }
+
+    @Test
+    fun testNewCoroutineContextDispatcher() {
+        fun newContextDispatcher(c1: CoroutineContext, c2: CoroutineContext) =
+            ContextScope(c1).newCoroutineContext(c2)[ContinuationInterceptor]
+
+        assertSame(Dispatchers.Default, newContextDispatcher(EmptyCoroutineContext, EmptyCoroutineContext))
+        assertSame(Dispatchers.Default, newContextDispatcher(EmptyCoroutineContext, Dispatchers.Default))
+        assertSame(Dispatchers.Default, newContextDispatcher(Dispatchers.Default, EmptyCoroutineContext))
+        assertSame(Dispatchers.Default, newContextDispatcher(Dispatchers.Default, Dispatchers.Default))
+        assertSame(Dispatchers.Default, newContextDispatcher(Dispatchers.Unconfined, Dispatchers.Default))
+        assertSame(Dispatchers.Unconfined, newContextDispatcher(Dispatchers.Default, Dispatchers.Unconfined))
+        assertSame(Dispatchers.Unconfined, newContextDispatcher(Dispatchers.Unconfined, Dispatchers.Unconfined))
+    }
+
     @Test
     fun testScopePlusContext() {
         assertSame(EmptyCoroutineContext, scopePlusContext(EmptyCoroutineContext, EmptyCoroutineContext))

kotlinx-coroutines-core/native/src/CoroutineContext.kt

@@ -31,8 +31,8 @@ internal actual val DefaultDelay: Delay = DefaultExecutor
 
 public actual fun CoroutineScope.newCoroutineContext(context: CoroutineContext): CoroutineContext {
     val combined = coroutineContext + context
-    return if (combined !== DefaultDelay && combined[ContinuationInterceptor] == null)
-        combined + (DefaultDelay as CoroutineContext.Element) else combined
+    return if (combined !== Dispatchers.Default && combined[ContinuationInterceptor] == null)
+        combined + Dispatchers.Default else combined
 }
 
 public actual fun CoroutineContext.newCoroutineContext(addedContext: CoroutineContext): CoroutineContext {