KT-860 ConcurrentModificationException in frontend

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

@@ -259,7 +259,7 @@ public class ConstraintSystemWithPriorities implements ConstraintSystem {
 
         assert constraintQueue.isEmpty() || unsolvedUnknowns.isEmpty() : constraintQueue + " " + unsolvedUnknowns;
 
-        for (TypeValue unknown : unsolvedUnknowns) {
+        for (TypeValue unknown : Sets.newLinkedHashSet(unsolvedUnknowns)) {
             if (!computeValueFor(unknown)) {
                 listener.error("Not enough data to compute value for ", unknown);
                 solution.registerError("Not enough data to compute value for " + unknown + ". Please, specify type arguments explicitly");

compiler/testData/diagnostics/tests/regressions/kt860.jet

@@ -0,0 +1,14 @@
+// KT-860 ConcurrentModificationException in frontend
+// +JDK
+
+namespace std.util
+
+import java.util.*
+
+fun <T, U: Collection<in T>> Iterator<T>.to(container: U) : U {
+    while(hasNext)
+        container.add(next())
+    return container
+}
+
+inline fun <T> Iterator<T>.toArrayList() = to(ArrayList<T>())