JS: fix behaviour of char-returning functions with multiple inheritance

See KT-19772

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsPlatformConfigurator.kt

@@ -35,7 +35,7 @@ object JsPlatformConfigurator : PlatformConfigurator(
         additionalDeclarationCheckers = listOf(
                 NativeInvokeChecker(), NativeGetterChecker(), NativeSetterChecker(),
                 JsNameChecker, JsModuleChecker, JsExternalFileChecker,
-                JsExternalChecker, JsInheritanceChecker,
+                JsExternalChecker, JsInheritanceChecker, JsMultipleInheritanceChecker,
                 JsRuntimeAnnotationChecker,
                 JsDynamicDeclarationChecker,
                 ExpectedActualDeclarationChecker

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/DefaultErrorMessagesJs.kt

@@ -104,6 +104,10 @@ private val DIAGNOSTIC_FACTORY_TO_RENDERER by lazy {
         put(ErrorsJs.EXTERNAL_CLASS_CONSTRUCTOR_PROPERTY_PARAMETER, "External class constructor cannot have a property parameter")
         put(ErrorsJs.CALL_TO_DEFINED_EXTERNALLY_FROM_NON_EXTERNAL_DECLARATION, "This property can only be used from external declarations")
 
+        put(ErrorsJs.WRONG_MULTIPLE_INHERITANCE,
+            "Can't apply multiple inheritance here, since it's impossible to generate bridge for system function {0}",
+            Renderers.DECLARATION_NAME_WITH_KIND)
+
         this
     }
 }

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/ErrorsJs.java

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.js.resolve.diagnostics;
 
 import com.intellij.psi.PsiElement;
+import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor;
 import org.jetbrains.kotlin.descriptors.DeclarationDescriptor;
 import org.jetbrains.kotlin.descriptors.FunctionDescriptor;
 import org.jetbrains.kotlin.diagnostics.*;
@@ -109,6 +110,9 @@ public interface ErrorsJs {
     DiagnosticFactory0<KtParameter> EXTERNAL_CLASS_CONSTRUCTOR_PROPERTY_PARAMETER = DiagnosticFactory0.create(ERROR);
     DiagnosticFactory0<PsiElement> CALL_TO_DEFINED_EXTERNALLY_FROM_NON_EXTERNAL_DECLARATION = DiagnosticFactory0.create(ERROR);
 
+    DiagnosticFactory1<PsiElement, CallableMemberDescriptor> WRONG_MULTIPLE_INHERITANCE =
+            DiagnosticFactory1.create(ERROR, DECLARATION_SIGNATURE_OR_DEFAULT);
+
     @SuppressWarnings("UnusedDeclaration")
     Object _initializer = new Object() {
         {

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/diagnostics/JsMultipleInheritanceChecker.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2010-2017 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.jetbrains.kotlin.js.resolve.diagnostics
+
+import org.jetbrains.kotlin.descriptors.CallableMemberDescriptor
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.descriptors.DeclarationDescriptor
+import org.jetbrains.kotlin.diagnostics.DiagnosticSink
+import org.jetbrains.kotlin.name.FqNameUnsafe
+import org.jetbrains.kotlin.psi.KtDeclaration
+import org.jetbrains.kotlin.resolve.BindingContext
+import org.jetbrains.kotlin.resolve.checkers.SimpleDeclarationChecker
+import org.jetbrains.kotlin.resolve.descriptorUtil.fqNameUnsafe
+
+object JsMultipleInheritanceChecker : SimpleDeclarationChecker {
+    private val fqNames = listOf(
+            FqNameUnsafe("kotlin.CharSequence.get"),
+            FqNameUnsafe("kotlin.collections.CharIterator.nextChar")
+    )
+    private val simpleNames = fqNames.mapTo(mutableSetOf()) { it.shortName() }
+
+    override fun check(
+            declaration: KtDeclaration, descriptor: DeclarationDescriptor,
+            diagnosticHolder: DiagnosticSink, bindingContext: BindingContext
+    ) {
+        if (descriptor !is ClassDescriptor) return
+
+        for (callable in descriptor.unsubstitutedMemberScope.getContributedDescriptors { it in simpleNames }
+                .filterIsInstance<CallableMemberDescriptor>()) {
+            if (callable.overriddenDescriptors.size > 1 && callable.overriddenDescriptors.any { it.fqNameUnsafe in fqNames }) {
+                diagnosticHolder.report(ErrorsJs.WRONG_MULTIPLE_INHERITANCE.on(declaration, callable))
+            }
+        }
+    }
+}