Delegation is only allowed if the target type is a trait

compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Errors.java

@@ -141,6 +141,7 @@ public interface Errors {
     SimpleDiagnosticFactory MANY_CLASS_OBJECTS = SimpleDiagnosticFactory.create(ERROR, "Only one class object is allowed per class");
     SimpleDiagnosticFactory CLASS_OBJECT_NOT_ALLOWED = SimpleDiagnosticFactory.create(ERROR, "A class object is not allowed here");
     SimpleDiagnosticFactory DELEGATION_IN_TRAIT = SimpleDiagnosticFactory.create(ERROR, "Traits cannot use delegation");
+    SimpleDiagnosticFactory DELEGATION_NOT_TO_TRAIT = SimpleDiagnosticFactory.create(ERROR, "Only traits can be delegated to");
     SimpleDiagnosticFactory NO_CONSTRUCTOR = SimpleDiagnosticFactory.create(ERROR, "This class does not have a constructor");
     SimpleDiagnosticFactory NOT_A_CLASS = SimpleDiagnosticFactory.create(ERROR, "Not a class");
     SimpleDiagnosticFactory ILLEGAL_ESCAPE_SEQUENCE = SimpleDiagnosticFactory.create(ERROR, "Illegal escape sequence");

compiler/frontend/src/org/jetbrains/jet/lang/resolve/BodyResolver.java

@@ -136,6 +136,15 @@ public class BodyResolver {
                 }
                 JetType supertype = context.getTrace().getBindingContext().get(BindingContext.TYPE, specifier.getTypeReference());
                 recordSupertype(specifier.getTypeReference(), supertype);
+                if (supertype != null) {
+                    DeclarationDescriptor declarationDescriptor = supertype.getConstructor().getDeclarationDescriptor();
+                    if (declarationDescriptor instanceof ClassDescriptor) {
+                        ClassDescriptor classDescriptor = (ClassDescriptor) declarationDescriptor;
+                        if (classDescriptor.getKind() != ClassKind.TRAIT) {
+                            context.getTrace().report(DELEGATION_NOT_TO_TRAIT.on(specifier.getTypeReference()));
+                        }
+                    }
+                }
                 JetExpression delegateExpression = specifier.getDelegateExpression();
                 if (delegateExpression != null) {
                     JetScope scope = scopeForConstructor == null

idea/testData/checkerWithErrorTypes/quick/DelegationNotTotrait.jet

@@ -0,0 +1,13 @@
+open class Foo() {
+
+}
+
+class Barrr() : <!DELEGATION_NOT_TO_TRAIT!>Foo<!> by Foo() {}
+
+trait T {}
+
+class Br(t : T) : T by t {}
+
+open enum class E() {}
+
+class Test2(e : E) : <!DELEGATION_NOT_TO_TRAIT!>E<!> by e {}