Fix actual typealias TestResult = Promise<Unit>

Author: dkhalanskyjb

kotlinx-coroutines-core/js/test/TestBase.kt

@@ -10,8 +10,14 @@ public actual val isStressTest: Boolean = false
 public actual val stressTestMultiplier: Int = 1
 public actual val stressTestMultiplierSqrt: Int = 1
 
-@Suppress("ACTUAL_WITHOUT_EXPECT", "ACTUAL_TYPE_ALIAS_TO_CLASS_WITH_DECLARATION_SITE_VARIANCE")
-public actual typealias TestResult = Promise<Unit>
+@JsName("Promise")
+external class MyPromise {
+    fun then(onFulfilled: ((Unit) -> Unit), onRejected: ((Throwable) -> Unit)): MyPromise
+    fun then(onFulfilled: ((Unit) -> Unit)): MyPromise
+}
+
+/** Always a `Promise<Unit>` */
+public actual typealias TestResult = MyPromise
 
 public actual val isNative = false
 
@@ -131,7 +137,7 @@ public actual open class TestBase actual constructor() {
             check(actionIndex == 0 || finished) { "Expecting that 'finish(...)' was invoked, but it was not" }
         }
         lastTestPromise = result
-        return result
+        return result as MyPromise
     }
 }
 

kotlinx-coroutines-test/js/src/TestBuilders.kt

@@ -3,15 +3,15 @@
  */
 
 package kotlinx.coroutines.test
+
 import kotlinx.coroutines.*
-import kotlin.js.*
+import kotlinx.coroutines.test.internal.*
 
-@Suppress("ACTUAL_WITHOUT_EXPECT", "ACTUAL_TYPE_ALIAS_TO_CLASS_WITH_DECLARATION_SITE_VARIANCE")
-public actual typealias TestResult = Promise<Unit>
+public actual typealias TestResult = JsPromiseInterfaceForTesting
 
 internal actual fun createTestResult(testProcedure: suspend CoroutineScope.() -> Unit): TestResult =
     GlobalScope.promise {
         testProcedure()
-    }
+    } as JsPromiseInterfaceForTesting
 
-internal actual fun dumpCoroutines() { }
+internal actual fun dumpCoroutines() {}

kotlinx-coroutines-test/js/src/internal/JsPromiseInterfaceForTesting.kt

@@ -0,0 +1,23 @@
+/*
+ * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines.test.internal
+
+/* This is a declaration of JS's `Promise<Unit>`. We need to keep it a separate class, because
+`actual typealias TestResult = Promise<Unit>` fails: you can't instantiate an `expect class` with a typealias to
+a parametric class. So, we make a non-parametric class just for this. */
+/**
+ * @suppress
+ */
+@JsName("Promise")
+public external class JsPromiseInterfaceForTesting {
+    /**
+     * @suppress
+     */
+    public fun then(onFulfilled: ((Unit) -> Unit), onRejected: ((Throwable) -> Unit)): JsPromiseInterfaceForTesting
+    /**
+     * @suppress
+     */
+    public fun then(onFulfilled: ((Unit) -> Unit)): JsPromiseInterfaceForTesting
+}