stop resolution if ambiguity is found

compiler/frontend/src/org/jetbrains/jet/lang/resolve/calls/CallResolver.java

@@ -262,10 +262,12 @@ public class CallResolver {
         for (ResolutionTask<D> task : prioritizedTasks) {
             TemporaryBindingTrace temporaryTrace = TemporaryBindingTrace.create(context.trace);
             OverloadResolutionResultsImpl<D> results = performResolutionGuardedForExtraFunctionLiteralArguments(task.withTrace(temporaryTrace));
-            if (results.isSuccess()) {
+            if (results.isSuccess() || results.isAmbiguity()) {
                 temporaryTrace.commit();
 
-                debugInfo.set(ResolutionDebugInfo.RESULT, results.getResultingCall());
+                if (results.isSuccess()) {
+                    debugInfo.set(ResolutionDebugInfo.RESULT, results.getResultingCall());
+                }
 
                 return results;
             }

compiler/testData/diagnostics/tests/scopes/stopResolutionOnAmbiguity.jet

@@ -0,0 +1,20 @@
+package c
+
+trait B {
+    fun bar() {}
+}
+
+class C() {
+    fun bar() {
+    }
+}
+
+fun test(a : Any?) {
+    if (a is B) {
+        if (a is C) {
+            a.<!OVERLOAD_RESOLUTION_AMBIGUITY!>bar()<!>;
+        }
+    }
+}
+
+fun Any?.bar() {}