Fix casts to type parameters with multiple bounds (#2891)

Fix #2888

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/TypeOperatorLowering.kt

@@ -61,7 +61,7 @@ private class TypeOperatorTransformer(val context: CommonBackendContext, val fun
         return when (classifier) {
             is IrClassSymbol -> this
             is IrTypeParameterSymbol -> {
-                val upperBound = classifier.owner.superTypes.singleOrNull() ?:
+                val upperBound = classifier.owner.superTypes.firstOrNull() ?:
                         TODO("${classifier.descriptor} : ${classifier.descriptor.upperBounds}")
 
                 if (this.hasQuestionMark) {

backend.native/tests/build.gradle

@@ -574,6 +574,12 @@ task unchecked_cast3(type: RunKonanTest) {
     source = "codegen/basics/unchecked_cast3.kt"
 }
 
+task unchecked_cast4(type: RunKonanTest) {
+    expectedFail = (project.testTarget == 'wasm32') // Uses exceptions.
+    goldValue = "Ok\n"
+    source = "codegen/basics/unchecked_cast4.kt"
+}
+
 task null_check(type: RunKonanTest) {
     source = "codegen/basics/null_check.kt"
 }

backend.native/tests/codegen/basics/unchecked_cast4.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the LICENSE file.
+ */
+
+package codegen.basics.unchecked_cast4
+
+import kotlin.test.*
+
+@Test
+fun runTest() {
+    CI1I2().uncheckedCast<CI1I2>()
+    CI1I2().uncheckedCast<OtherCI1I2>()
+
+    assertFailsWith<ClassCastException> {
+        Any().uncheckedCast<CI1I2>()
+    }
+
+    println("Ok")
+}
+
+fun <R : C> Any?.uncheckedCast() where R : I1, R : I2 {
+    this as R
+}
+
+interface I1
+interface I2
+open class C
+
+class CI1I2 : C(), I1, I2
+class OtherCI1I2 : C(), I1, I2