Improve the error message for when the Main dispatcher is missing

Author: dkhalanskyjb

kotlinx-coroutines-test/jvm/src/internal/TestMainDispatcherJvm.kt

@@ -10,7 +10,21 @@ internal class TestMainDispatcherFactory : MainDispatcherFactory {
         val secondBestFactory = otherFactories.maxByOrNull { it.loadPriority } ?: MissingMainCoroutineDispatcherFactory
         /* Do not immediately create the alternative dispatcher, as with `SUPPORT_MISSING` set to `false`,
         it will throw an exception. Instead, create it lazily. */
-        return TestMainDispatcher({ secondBestFactory.tryCreateDispatcher(otherFactories) })
+        return TestMainDispatcher({
+            val dispatcher = try {
+                secondBestFactory.tryCreateDispatcher(otherFactories)
+            } catch (e: Throwable) {
+                reportMissingMainCoroutineDispatcher(e)
+            }
+            if (dispatcher.isMissing()) {
+                reportMissingMainCoroutineDispatcher(runCatching {
+                    // attempt to dispatch something to the missing dispatcher to trigger the exception.
+                    dispatcher.dispatch(dispatcher, Runnable { })
+                }.exceptionOrNull()) // can not be null, but it does not matter.
+            } else {
+                dispatcher
+            }
+        })
     }
 
     /**
@@ -26,3 +40,13 @@ internal actual fun Dispatchers.getTestMainDispatcher(): TestMainDispatcher {
     require(mainDispatcher is TestMainDispatcher) { "TestMainDispatcher is not set as main dispatcher, have $mainDispatcher instead." }
     return mainDispatcher
 }
+
+private fun reportMissingMainCoroutineDispatcher(e: Throwable? = null): Nothing {
+    throw IllegalStateException(
+        "Dispatchers.Main was accessed when the platform dispatcher was absent " +
+            "and the test dispatcher was unset. Please make sure that Dispatchers.setMain() is called " +
+            "before accessing Dispatchers.Main and that Dispatchers.Main is not accessed after " +
+            "Dispatchers.resetMain().",
+        e
+    )
+}