[JS IR BE] Support companion objects in external interfaces

These objects are referenced but value is not actually used.
Replace access to these references with null literals.

compiler/ir/backend.js/src/org/jetbrains/kotlin/ir/backend/js/transformers/irToJs/IrElementToJsExpressionTransformer.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -95,6 +95,14 @@ class IrElementToJsExpressionTransformer : BaseIrElementToJsNodeTransformer<JsEx
         val obj = expression.symbol.owner
         assert(obj.kind == ClassKind.OBJECT)
         assert(obj.isEffectivelyExternal()) { "Non external IrGetObjectValue must be lowered" }
+
+        // External interfaces cannot normally have companion objects.
+        // However, stdlib uses them to simulate string literal unions
+        // TODO: Stop abusing this tech
+        if (obj.isCompanion && obj.parentAsClass.isInterface) {
+            return JsNullLiteral()
+        }
+
         return context.getRefForExternalClass(obj)
     }
 

js/js.tests/test/org/jetbrains/kotlin/js/test/ir/semantics/IrBoxJsTestGenerated.java

@@ -6479,9 +6479,7 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
         }
 
         public void testAllFilesPresentInPropertyAccess() throws Exception {
-            KotlinTestUtils
-                    .assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("js/js.translator/testData/box/propertyAccess"),
-                                                                 Pattern.compile("^([^_](.+))\\.kt$"), null, TargetBackend.JS_IR, true);
+            KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("js/js.translator/testData/box/propertyAccess"), Pattern.compile("^([^_](.+))\\.kt$"), null, TargetBackend.JS_IR, true);
         }
 
         @TestMetadata("classUsesPackageProperties.kt")
@@ -6816,6 +6814,11 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
             KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("js/js.translator/testData/box/regression"), Pattern.compile("^([^_](.+))\\.kt$"), null, TargetBackend.JS_IR, true);
         }
 
+        @TestMetadata("companionObjectInExternalInterface.kt")
+        public void testCompanionObjectInExternalInterface() throws Exception {
+            runTest("js/js.translator/testData/box/regression/companionObjectInExternalInterface.kt");
+        }
+
         @TestMetadata("enumEntryInitOrder.kt")
         public void testEnumEntryInitOrder() throws Exception {
             runTest("js/js.translator/testData/box/regression/enumEntryInitOrder.kt");

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/BoxJsTestGenerated.java

@@ -6504,9 +6504,7 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
         }
 
         public void testAllFilesPresentInPropertyAccess() throws Exception {
-            KotlinTestUtils
-                    .assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("js/js.translator/testData/box/propertyAccess"),
-                                                                 Pattern.compile("^([^_](.+))\\.kt$"), null, TargetBackend.JS, true);
+            KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("js/js.translator/testData/box/propertyAccess"), Pattern.compile("^([^_](.+))\\.kt$"), null, TargetBackend.JS, true);
         }
 
         @TestMetadata("classUsesPackageProperties.kt")
@@ -6871,6 +6869,11 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
             KotlinTestUtils.assertAllTestsPresentByMetadataWithExcluded(this.getClass(), new File("js/js.translator/testData/box/regression"), Pattern.compile("^([^_](.+))\\.kt$"), null, TargetBackend.JS, true);
         }
 
+        @TestMetadata("companionObjectInExternalInterface.kt")
+        public void testCompanionObjectInExternalInterface() throws Exception {
+            runTest("js/js.translator/testData/box/regression/companionObjectInExternalInterface.kt");
+        }
+
         @TestMetadata("enumEntryInitOrder.kt")
         public void testEnumEntryInitOrder() throws Exception {
             runTest("js/js.translator/testData/box/regression/enumEntryInitOrder.kt");

js/js.translator/testData/box/regression/companionObjectInExternalInterface.kt

@@ -0,0 +1,17 @@
+// EXPECTED_REACHABLE_NODES: 1303
+
+// This hack is used in org.w3c.* part of standard library to represent unions of Strings
+// Test that we are not actually trying to access nonexistent companion object
+
+@Suppress("NESTED_CLASS_IN_EXTERNAL_INTERFACE")
+public external interface I {
+    companion object
+}
+public inline val I.Companion.O: I get() = "O".asDynamic().unsafeCast<I>()
+public inline val I.Companion.K: I get() = "K".asDynamic().unsafeCast<I>()
+
+fun box(): String {
+    if (I.O != I.O) return "Fail 1"
+    if (I.O == I.K) return "Fail 2"
+    return I.O.unsafeCast<String>() + I.K.unsafeCast<String>()
+}