Test invariant that IDEA evaluator depends on (#2525)

Fixes #2522

Author: qwwdfsad

kotlinx-coroutines-core/jvm/test/IntellijIdeaDebuggerEvaluatorCompatibilityTest.kt

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.coroutines
+
+import org.junit.Test
+import kotlin.coroutines.*
+import kotlin.test.*
+
+class IntellijIdeaDebuggerEvaluatorCompatibilityTest {
+
+    /*
+     * This test verifies that our CoroutineScope is accessible to IDEA debugger.
+     *
+     * Consider the following scenario:
+     * ```
+     * runBlocking<Unit> { // this: CoroutineScope
+     *     println("runBlocking")
+     * }
+     * ```
+     * user puts breakpoint to `println` line, opens "Evaluate" window
+     * and executes `launch { println("launch") }`. They (obviously) expect it to work, but
+     * it won't: `{}` in `runBlocking` is `SuspendLambda` and `this` is an unused implicit receiver
+     * that is removed by the compiler (because it's unused).
+     *
+     * But we still want to provide consistent user experience for functions with `CoroutineScope` receiver,
+     * for that IDEA debugger tries to retrieve the scope via `kotlin.coroutines.coroutineContext[Job] as? CoroutineScope`
+     * and with this test we're fixing this behaviour.
+     *
+     * Note that this behaviour is not carved in stone: IDEA fallbacks to `kotlin.coroutines.coroutineContext` for the context if necessary.
+     */
+
+    @Test
+    fun testScopeIsAccessible() = runBlocking<Unit> {
+        verify()
+
+        withContext(Job()) {
+            verify()
+        }
+
+        coroutineScope {
+            verify()
+        }
+
+        supervisorScope {
+            verify()
+        }
+
+    }
+
+    private suspend fun verify() {
+        val ctx = coroutineContext
+        assertTrue { ctx.job is CoroutineScope }
+    }
+}