Change importing rules for objects:

Allow importing classes from object, prohibit import-on-demand from objects
It's unclear what import-on-demand from object should mean so we prohibit it for now

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/Errors.java

@@ -100,6 +100,7 @@ public interface Errors {
     // Imports
 
     DiagnosticFactory1<JetSimpleNameExpression, DeclarationDescriptor> CANNOT_IMPORT_FROM_ELEMENT = DiagnosticFactory1.create(ERROR);
+    DiagnosticFactory1<JetSimpleNameExpression, ClassDescriptor> CANNOT_IMPORT_ON_DEMAND_FROM_SINGLETON = DiagnosticFactory1.create(ERROR);
     DiagnosticFactory1<JetSimpleNameExpression, DeclarationDescriptor> CANNOT_BE_IMPORTED = DiagnosticFactory1.create(ERROR);
 
     DiagnosticFactory1<JetExpression, String> CONFLICTING_IMPORT = DiagnosticFactory1.create(ERROR);

compiler/frontend/src/org/jetbrains/kotlin/diagnostics/rendering/DefaultErrorMessages.java

@@ -166,6 +166,7 @@ public class DefaultErrorMessages {
         MAP.put(EXPRESSION_EXPECTED_PACKAGE_FOUND, "Expression expected, but a package name found");
 
         MAP.put(CANNOT_IMPORT_FROM_ELEMENT, "Cannot import from ''{0}''", NAME);
+        MAP.put(CANNOT_IMPORT_ON_DEMAND_FROM_SINGLETON, "Cannot import-on-demand from object ''{0}''", NAME);
         MAP.put(CANNOT_BE_IMPORTED, "Cannot import ''{0}'', functions and properties can be imported only from packages", NAME);
         MAP.put(CONFLICTING_IMPORT, "Conflicting import, imported name ''{0}'' is ambiguous", STRING);
         MAP.put(PLATFORM_CLASS_MAPPED_TO_KOTLIN, "This class shouldn''t be used in Kotlin. Use {0} instead.", CLASSES_OR_SEPARATED);

compiler/frontend/src/org/jetbrains/kotlin/resolve/Importer.kt

@@ -34,7 +34,7 @@ public class Importer {
         if (descriptor is PackageViewDescriptor) {
             allUnderImportScopes.add(NoSubpackagesInPackageScope(descriptor))
         }
-        else if (descriptor is ClassDescriptor && descriptor.getKind() != ClassKind.OBJECT) {
+        else if (descriptor is ClassDescriptor && QualifiedExpressionResolver.canAllUnderImportFromClass(descriptor)) {
             allUnderImportScopes.add(descriptor.getStaticScope())
             allUnderImportScopes.add(descriptor.getUnsubstitutedInnerClassesScope())
 

compiler/frontend/src/org/jetbrains/kotlin/resolve/QualifiedExpressionResolver.java

@@ -54,6 +54,25 @@ public class QualifiedExpressionResolver {
         EVERYTHING
     }
 
+    public static boolean canAllUnderImportFrom(@NotNull Collection<DeclarationDescriptor> descriptors) {
+        if (descriptors.isEmpty()) {
+            return true;
+        }
+        for (DeclarationDescriptor descriptor : descriptors) {
+            if (!(descriptor instanceof ClassDescriptor)) {
+                return true;
+            }
+            if (canAllUnderImportFromClass((ClassDescriptor) descriptor)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    public static boolean canAllUnderImportFromClass(@NotNull ClassDescriptor descriptor) {
+        return !descriptor.getKind().isSingleton();
+    }
+
     @NotNull
     public Collection<DeclarationDescriptor> processImportReference(
             @NotNull JetImportDirective importDirective,
@@ -88,6 +107,10 @@ public class QualifiedExpressionResolver {
 
         JetSimpleNameExpression referenceExpression = JetPsiUtil.getLastReference(importedReference);
         if (importDirective.isAllUnder()) {
+            if (!canAllUnderImportFrom(descriptors) && referenceExpression != null) {
+                trace.report(CANNOT_IMPORT_ON_DEMAND_FROM_SINGLETON.on(referenceExpression, (ClassDescriptor) descriptors.iterator().next()));
+            }
+
             if (referenceExpression == null || !canImportMembersFrom(descriptors, referenceExpression, trace, lookupMode)) {
                 return Collections.emptyList();
             }
@@ -150,7 +173,7 @@ public class QualifiedExpressionResolver {
         if (descriptor instanceof PackageViewDescriptor) {
             return true;
         }
-        if (descriptor instanceof ClassDescriptor && !((ClassDescriptor) descriptor).getKind().isSingleton()) {
+        if (descriptor instanceof ClassDescriptor) {
             return true;
         }
         trace.report(CANNOT_IMPORT_FROM_ELEMENT.on(reference, descriptor));