Type capabilities introduced to facilitate extensibility of the type representation

This approach: returning adapter objects by class is crucial, because it is delegation-proof.
Simply extending interfaces in particular implementations fails when we wrap types in DelegatedType subclasses

core/descriptors/src/org/jetbrains/jet/lang/types/AbstractJetType.java

@@ -16,12 +16,24 @@
 
 package org.jetbrains.jet.lang.types;
 
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.jetbrains.jet.lang.types.checker.JetTypeChecker;
 
 import java.util.Iterator;
 import java.util.List;
 
 public abstract class AbstractJetType extends InflexibleType {
+    @Nullable
+    @Override
+    public <T extends TypeCapability> T getCapability(@NotNull Class<T> capabilityClass) {
+        if (capabilityClass.isInstance(this)) {
+            //noinspection unchecked
+            return (T) this;
+        }
+        return null;
+    }
+
     @Override
     public final int hashCode() {
         int result = getConstructor().hashCode();

core/descriptors/src/org/jetbrains/jet/lang/types/DelegatingType.java

@@ -17,6 +17,7 @@
 package org.jetbrains.jet.lang.types;
 
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.jetbrains.jet.lang.descriptors.annotations.Annotations;
 import org.jetbrains.jet.lang.resolve.scopes.JetScope;
 
@@ -76,6 +77,16 @@ public abstract class DelegatingType implements JetType {
         return getDelegate().getAnnotations();
     }
 
+    @Override
+    @Nullable
+    public <T extends TypeCapability> T getCapability(@NotNull Class<T> capabilityClass) {
+        if (capabilityClass.isInstance(this)) {
+            //noinspection unchecked
+            return (T) this;
+        }
+        return getDelegate().getCapability(capabilityClass);
+    }
+
     @Override
     public int hashCode() {
         return getDelegate().hashCode();

core/descriptors/src/org/jetbrains/jet/lang/types/ErrorUtils.java

@@ -421,6 +421,12 @@ public class ErrorUtils {
             return Annotations.EMPTY;
         }
 
+        @Nullable
+        @Override
+        public <T extends TypeCapability> T getCapability(@NotNull Class<T> capabilityClass) {
+            return null;
+        }
+
         @Override
         public String toString() {
             return constructor.toString();

core/descriptors/src/org/jetbrains/jet/lang/types/JetType.java

@@ -17,6 +17,7 @@
 package org.jetbrains.jet.lang.types;
 
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.jetbrains.annotations.ReadOnly;
 import org.jetbrains.jet.lang.descriptors.annotations.Annotated;
 import org.jetbrains.jet.lang.resolve.scopes.JetScope;
@@ -53,4 +54,7 @@ public interface JetType extends Annotated {
 
     // isFlexible() == false <=> getLowerBound() == getUpperBound()
     boolean isFlexible();
+
+    @Nullable
+    <T extends TypeCapability> T getCapability(@NotNull Class<T> capabilityClass);
 }

core/descriptors/src/org/jetbrains/jet/lang/types/TypeCapabilities.kt

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2010-2014 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.jet.lang.types
+
+import org.jetbrains.jet.lang.descriptors.TypeParameterDescriptor
+
+public trait TypeCapability

core/descriptors/src/org/jetbrains/jet/lang/types/TypeUtils.java

@@ -85,6 +85,12 @@ public class TypeUtils {
             throw new IllegalStateException(name);
         }
 
+        @Nullable
+        @Override
+        public <T extends TypeCapability> T getCapability(@NotNull Class<T> capabilityClass) {
+            return null;
+        }
+
         @Override
         public String toString() {
             return name;