Report warning on 'Singleton.javaClass'

As a temporary measure before 'javaClass' property is deprecated and removed.
Getting the Java class of a singleton this way is confusing and most of the
time is a failed attempt to get the Java class of a class with a companion
object (e.g. 'Int.javaClass')

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/calls/checkers/JavaClassOnCompanionChecker.kt

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2010-2015 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.resolve.jvm.calls.checkers
+
+import org.jetbrains.kotlin.descriptors.CallableDescriptor
+import org.jetbrains.kotlin.descriptors.ClassDescriptor
+import org.jetbrains.kotlin.descriptors.PackageFragmentDescriptor
+import org.jetbrains.kotlin.descriptors.PropertyDescriptor
+import org.jetbrains.kotlin.descriptors.annotations.Annotations
+import org.jetbrains.kotlin.resolve.calls.checkers.CallChecker
+import org.jetbrains.kotlin.resolve.calls.context.BasicCallResolutionContext
+import org.jetbrains.kotlin.resolve.calls.model.ResolvedCall
+import org.jetbrains.kotlin.resolve.jvm.diagnostics.ErrorsJvm
+import org.jetbrains.kotlin.types.JetTypeImpl
+import org.jetbrains.kotlin.types.TypeProjectionImpl
+
+public class JavaClassOnCompanionChecker : CallChecker {
+    override fun <F : CallableDescriptor> check(resolvedCall: ResolvedCall<F>, context: BasicCallResolutionContext) {
+        val descriptor = resolvedCall.resultingDescriptor
+        if (descriptor !is PropertyDescriptor || descriptor.name.asString() != "javaClass") return
+
+        val container = descriptor.containingDeclaration
+        if (container !is PackageFragmentDescriptor || container.fqName.asString() != "kotlin") return
+
+        val actualType = descriptor.type
+
+        val companionObject = actualType.arguments.singleOrNull()?.type?.constructor?.declarationDescriptor as? ClassDescriptor ?: return
+        if (companionObject.isCompanionObject) {
+            val containingClass = companionObject.containingDeclaration as ClassDescriptor
+            val javaLangClass = actualType.constructor.declarationDescriptor as? ClassDescriptor ?: return
+            val expectedType = JetTypeImpl.create(
+                    Annotations.EMPTY, javaLangClass, actualType.isMarkedNullable,
+                    listOf(TypeProjectionImpl(containingClass.defaultType))
+            )
+            context.trace.report(ErrorsJvm.JAVA_CLASS_ON_COMPANION.on(resolvedCall.call.callElement, actualType, expectedType))
+        }
+    }
+}

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/diagnostics/DefaultErrorMessagesJvm.java

@@ -75,6 +75,12 @@ public class DefaultErrorMessagesJvm implements DefaultErrorMessages.Extension {
         MAP.put(ErrorsJvm.WHEN_ENUM_CAN_BE_NULL_IN_JAVA, "Enum argument can be null in Java, but exhaustive when contains no null branch");
 
         MAP.put(ErrorsJvm.INAPPLICABLE_PUBLIC_FIELD, "publicField annotation is not applicable to this declaration");
+
+        MAP.put(ErrorsJvm.JAVA_CLASS_ON_COMPANION,
+                "The resulting type of this ''javaClass'' call is {0} and not {1}. " +
+                "Please use the more clear ''::class.java'' syntax to avoid confusion",
+                Renderers.RENDER_TYPE, Renderers.RENDER_TYPE
+        );
     }
 
     @NotNull

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/diagnostics/ErrorsJvm.java

@@ -27,6 +27,7 @@ import org.jetbrains.kotlin.psi.JetAnnotationEntry;
 import org.jetbrains.kotlin.psi.JetDeclaration;
 import org.jetbrains.kotlin.psi.JetElement;
 import org.jetbrains.kotlin.psi.JetExpression;
+import org.jetbrains.kotlin.types.JetType;
 
 import static org.jetbrains.kotlin.diagnostics.PositioningStrategies.*;
 import static org.jetbrains.kotlin.diagnostics.Severity.ERROR;
@@ -65,6 +66,8 @@ public interface ErrorsJvm {
 
     DiagnosticFactory0<JetElement> NO_REFLECTION_IN_CLASS_PATH = DiagnosticFactory0.create(WARNING);
 
+    DiagnosticFactory2<JetElement, JetType, JetType> JAVA_CLASS_ON_COMPANION = DiagnosticFactory2.create(WARNING);
+
     enum NullabilityInformationSource {
         KOTLIN {
             @NotNull

compiler/frontend.java/src/org/jetbrains/kotlin/resolve/jvm/platform/JvmPlatformConfigurator.kt

@@ -50,6 +50,7 @@ import org.jetbrains.kotlin.resolve.calls.smartcasts.DataFlowValueFactory
 import org.jetbrains.kotlin.resolve.calls.smartcasts.Nullability
 import org.jetbrains.kotlin.resolve.descriptorUtil.getAnnotationRetention
 import org.jetbrains.kotlin.resolve.descriptorUtil.isRepeatableAnnotation
+import org.jetbrains.kotlin.resolve.jvm.calls.checkers.JavaClassOnCompanionChecker
 import org.jetbrains.kotlin.resolve.jvm.calls.checkers.NeedSyntheticChecker
 import org.jetbrains.kotlin.resolve.jvm.calls.checkers.ReflectionAPICallChecker
 import org.jetbrains.kotlin.resolve.jvm.calls.checkers.TraitDefaultMethodCallChecker
@@ -76,7 +77,8 @@ public object JvmPlatformConfigurator : PlatformConfigurator(
                 NeedSyntheticChecker(),
                 JavaAnnotationCallChecker(),
                 JavaAnnotationMethodCallChecker(),
-                TraitDefaultMethodCallChecker()
+                TraitDefaultMethodCallChecker(),
+                JavaClassOnCompanionChecker()
         ),
 
         additionalTypeCheckers = listOf(