diff --git a/compiler/backend/src/org/jetbrains/jet/codegen/StackValue.java b/compiler/backend/src/org/jetbrains/jet/codegen/StackValue.java
index cafee41d2cc..547a90a7731 100644
--- a/compiler/backend/src/org/jetbrains/jet/codegen/StackValue.java
+++ b/compiler/backend/src/org/jetbrains/jet/codegen/StackValue.java
@@ -556,7 +556,7 @@ public abstract class StackValue {
         }
 
         public int receiverSize() {
-            if(isStandardStack(resolvedGetCall) && isStandardStack(resolvedSetCall)) {
+            if(isStandardStack(resolvedGetCall, 1) && isStandardStack(resolvedSetCall, 2)) {
                 return 2;
             }
             else {
@@ -566,7 +566,7 @@ public abstract class StackValue {
 
         @Override
         public void dupReceiver(InstructionAdapter v) {
-            if(isStandardStack(resolvedGetCall) && isStandardStack(resolvedSetCall)) {
+            if(isStandardStack(resolvedGetCall, 1) && isStandardStack(resolvedSetCall, 2)) {
                 v.dup2();   // collection and index
             }
             else {
@@ -702,7 +702,7 @@ public abstract class StackValue {
             }
         }
 
-        private boolean isStandardStack(ResolvedCall call) {
+        private boolean isStandardStack(ResolvedCall call, int valueParamsSize) {
             if(call == null)
                 return true;
 
@@ -710,12 +710,15 @@ public abstract class StackValue {
                 if(typeParameterDescriptor.isReified())
                     return false;
             }
-            
-            if(call.getResultingDescriptor().getValueParameters().size() != 1)
+
+            List<ValueParameterDescriptor> valueParameters = call.getResultingDescriptor().getValueParameters();
+            if(valueParameters.size() != valueParamsSize)
                 return false;
 
-            if(codegen.typeMapper.mapType(call.getResultingDescriptor().getValueParameters().get(0).getOutType()).getSize() != 1)
-                return false;
+            for (ValueParameterDescriptor valueParameter : valueParameters) {
+                if (codegen.typeMapper.mapType(valueParameter.getOutType()).getSize() != 1)
+                    return false;
+            }
 
             if(call.getThisObject().exists()) {
                 if(call.getReceiverArgument().exists())
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/cfg/JetFlowInformationProvider.java b/compiler/frontend/src/org/jetbrains/jet/lang/cfg/JetFlowInformationProvider.java
index dd16bbc7f18..4796325484e 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/cfg/JetFlowInformationProvider.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/cfg/JetFlowInformationProvider.java
@@ -310,6 +310,16 @@ public class JetFlowInformationProvider {
                         hasReassignMethodReturningUnit = true;
                     }
                 }
+                if (descriptor == null) {
+                    Collection<? extends DeclarationDescriptor> descriptors = trace.get(BindingContext.AMBIGUOUS_REFERENCE_TARGET, operationReference);
+                    if (descriptors != null) {
+                        for (DeclarationDescriptor referenceDescriptor : descriptors) {
+                            if (JetStandardClasses.isUnit(((FunctionDescriptor) referenceDescriptor).getReturnType())) {
+                                hasReassignMethodReturningUnit = true;
+                            }
+                        }
+                    }
+                }
             }
             if (!hasReassignMethodReturningUnit) {
                 trace.report(Errors.VAL_REASSIGNMENT.on(expression, variableDescriptor));
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Errors.java b/compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Errors.java
index 99cdb406f14..eee0c5de5e1 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Errors.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/diagnostics/Errors.java
@@ -199,6 +199,15 @@ public interface Errors {
     SimpleDiagnosticFactory NO_GET_METHOD = SimpleDiagnosticFactory.create(ERROR, "No get method providing array access");
     SimpleDiagnosticFactory NO_SET_METHOD = SimpleDiagnosticFactory.create(ERROR, "No set method providing array access");
 
+    SimpleDiagnosticFactory INC_DEC_SHOULD_NOT_RETURN_UNIT = SimpleDiagnosticFactory.create(ERROR, "Functions inc(), dec() shouldn't return Unit to be used by operators ++, --");
+    ParameterizedDiagnosticFactory2<DeclarationDescriptor, JetSimpleNameExpression> ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT = new ParameterizedDiagnosticFactory2<DeclarationDescriptor, JetSimpleNameExpression>(ERROR, "Function ''{0}'' should return Unit to be used by corresponding operator ''{1}''", NAME) {
+        @Override
+        protected String makeMessageForB(@NotNull JetSimpleNameExpression expression) {
+            return expression.getText();
+        }
+    };
+    AmbiguousDescriptorDiagnosticFactory ASSIGN_OPERATOR_AMBIGUITY = AmbiguousDescriptorDiagnosticFactory.create("Assignment operators ambiguity: {0}");
+
     SimpleDiagnosticFactory EQUALS_MISSING = SimpleDiagnosticFactory.create(ERROR, "No method 'equals(Any?) : Boolean' available");
     SimpleDiagnosticFactory ASSIGNMENT_IN_EXPRESSION_CONTEXT = SimpleDiagnosticFactory.create(ERROR, "Assignments are not expressions, and only expressions are allowed in this context");
     SimpleDiagnosticFactory NAMESPACE_IS_NOT_AN_EXPRESSION = SimpleDiagnosticFactory.create(ERROR, "'namespace' is not an expression, it can only be used on the left-hand side of a dot ('.')");
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/resolve/calls/OverloadResolutionResultsUtil.java b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/calls/OverloadResolutionResultsUtil.java
new file mode 100644
index 00000000000..a480b2c6ac5
--- /dev/null
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/resolve/calls/OverloadResolutionResultsUtil.java
@@ -0,0 +1,30 @@
+package org.jetbrains.jet.lang.resolve.calls;
+
+import com.google.common.collect.Lists;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+import org.jetbrains.jet.lang.descriptors.CallableDescriptor;
+import org.jetbrains.jet.lang.types.JetType;
+
+import java.util.Collection;
+
+/**
+ * @author svtk
+ */
+public class OverloadResolutionResultsUtil {
+    @NotNull
+    public static <D extends CallableDescriptor> OverloadResolutionResults<D> ambiguity(OverloadResolutionResults<D> results1, OverloadResolutionResults<D> results2) {
+        Collection<ResolvedCallImpl<D>> resultingCalls = Lists.newArrayList();
+        resultingCalls.addAll((Collection<ResolvedCallImpl<D>>) results1.getResultingCalls());
+        resultingCalls.addAll((Collection<ResolvedCallImpl<D>>) results2.getResultingCalls());
+        return OverloadResolutionResultsImpl.ambiguity(resultingCalls);
+    }
+
+    @Nullable
+    public static <D extends CallableDescriptor> JetType getResultType(OverloadResolutionResults<D> results) {
+        if (results.isSuccess()) {
+            return results.getResultingDescriptor().getReturnType();
+        }
+        return null;
+    }
+}
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/BasicExpressionTypingVisitor.java b/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/BasicExpressionTypingVisitor.java
index acf7b2db7fa..b18bc19571c 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/BasicExpressionTypingVisitor.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/BasicExpressionTypingVisitor.java
@@ -14,6 +14,7 @@ import org.jetbrains.jet.lang.psi.*;
 import org.jetbrains.jet.lang.resolve.*;
 import org.jetbrains.jet.lang.resolve.calls.CallMaker;
 import org.jetbrains.jet.lang.resolve.calls.OverloadResolutionResults;
+import org.jetbrains.jet.lang.resolve.calls.OverloadResolutionResultsUtil;
 import org.jetbrains.jet.lang.resolve.calls.autocasts.DataFlowInfo;
 import org.jetbrains.jet.lang.resolve.constants.*;
 import org.jetbrains.jet.lang.resolve.constants.StringValue;
@@ -27,6 +28,7 @@ import org.jetbrains.jet.lang.types.*;
 import org.jetbrains.jet.lang.types.checker.JetTypeChecker;
 import org.jetbrains.jet.lexer.JetTokens;
 
+import javax.naming.ldap.ExtendedResponse;
 import java.util.*;
 
 import static org.jetbrains.jet.lang.diagnostics.Errors.*;
@@ -661,24 +663,19 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
             context.trace.report(UNSUPPORTED.on(operationSign, "visitUnaryExpression"));
             return null;
         }
-        JetType type = null;
+        TemporaryBindingTrace temporaryTrace = TemporaryBindingTrace.create(context.trace);
+        BindingTrace initialTrace = context.trace;
+        context = context.replaceBindingTrace(temporaryTrace);
+        JetType type = facade.getType(baseExpression, context.replaceExpectedType(NO_EXPECTED_TYPE));
+        if (type == null) {
+            temporaryTrace.commit();
+            return null;
+        }
 
         if ((operationType == JetTokens.PLUSPLUS || operationType == JetTokens.MINUSMINUS) && baseExpression instanceof JetArrayAccessExpression) {
-            TemporaryBindingTrace temporaryTrace = TemporaryBindingTrace.create(context.trace);
-            JetType expressionType = facade.getType(baseExpression, context.replaceExpectedType(NO_EXPECTED_TYPE).replaceBindingTrace(temporaryTrace));
-            if (expressionType != null) {
-                JetExpression stubExpression = ExpressionTypingUtils.createStubExpressionOfNecessaryType(baseExpression.getProject(), expressionType, context.trace);
-                type = resolveArrayAccessToLValue((JetArrayAccessExpression) baseExpression, stubExpression, context, true);
-            }
-            else {
-                temporaryTrace.commit();
-                context.trace.report(UNRESOLVED_REFERENCE.on(operationSign));
-            }
+            JetExpression stubExpression = ExpressionTypingUtils.createStubExpressionOfNecessaryType(baseExpression.getProject(), type, context.trace);
+            resolveArrayAccessSetMethod((JetArrayAccessExpression) baseExpression, stubExpression, context.replaceExpectedType(NO_EXPECTED_TYPE).replaceBindingTrace(TemporaryBindingTrace.create(initialTrace)), context.trace);
         }
-        else {
-            type = facade.getType(baseExpression, context.replaceExpectedType(NO_EXPECTED_TYPE));
-        }
-        if (type == null) return null;
 
         ExpressionReceiver receiver = new ExpressionReceiver(baseExpression, type);
 
@@ -687,12 +684,16 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
                 expression.getOperationReference(),
                 name);
 
-        if (!functionDescriptor.isSuccess()) return null;
+        if (!functionDescriptor.isSuccess()) {
+            temporaryTrace.commit();
+            return null;
+        }
         JetType returnType = functionDescriptor.getResultingDescriptor().getReturnType();
         JetType result;
         if (operationType == JetTokens.PLUSPLUS || operationType == JetTokens.MINUSMINUS) {
             if (context.semanticServices.getTypeChecker().isSubtypeOf(returnType, JetStandardClasses.getUnitType())) {
                 result = JetStandardClasses.getUnitType();
+                context.trace.report(INC_DEC_SHOULD_NOT_RETURN_UNIT.on(operationSign));
             }
             else {
                 JetType receiverType = receiver.getType();
@@ -712,7 +713,7 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
         else {
             result = returnType;
         }
-
+        temporaryTrace.commit();
         return DataFlowUtils.checkType(result, expression, context);
     }
 
@@ -849,13 +850,13 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
     public boolean checkInExpression(JetElement callElement, @NotNull JetSimpleNameExpression operationSign, @Nullable JetExpression left, @NotNull JetExpression right, ExpressionTypingContext context) {
         String name = "contains";
         ExpressionReceiver receiver = safeGetExpressionReceiver(facade, right, context.replaceExpectedType(NO_EXPECTED_TYPE));
-        OverloadResolutionResults<FunctionDescriptor> functionDescriptor = context.resolveCallWithGivenNameToDescriptor(
+        OverloadResolutionResults<FunctionDescriptor> resolutionResult = context.resolveCallWithGivenNameToDescriptor(
                 CallMaker.makeCallWithExpressions(callElement, receiver, null, operationSign, Collections.singletonList(left)),
                 operationSign,
                 name);
-        JetType containsType = functionDescriptor.isSuccess() ? functionDescriptor.getResultingDescriptor().getReturnType() : null;
+        JetType containsType = OverloadResolutionResultsUtil.getResultType(resolutionResult);
         ensureBooleanResult(operationSign, name, containsType, context);
-        return functionDescriptor.isSuccess();
+        return resolutionResult.isSuccess();
     }
 
     private void ensureNonemptyIntersectionOfOperandTypes(JetBinaryExpression expression, ExpressionTypingContext context) {
@@ -892,38 +893,24 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
     }
 
     @Override
-    public JetType visitArrayAccessExpression(JetArrayAccessExpression expression, ExpressionTypingContext contextWithExpectedType) {
-        ExpressionTypingContext context = contextWithExpectedType.replaceExpectedType(NO_EXPECTED_TYPE);
-        JetExpression arrayExpression = expression.getArrayExpression();
-        ExpressionReceiver receiver = getExpressionReceiver(facade, arrayExpression, context.replaceScope(context.scope));
-
-        if (receiver != null) {
-            OverloadResolutionResults<FunctionDescriptor> results = context.resolveCallWithGivenName(
-                    CallMaker.makeCallWithExpressions(expression, receiver, null, expression, expression.getIndexExpressions()),
-                    expression,
-                    "get"
-            );
-            if (results.isSuccess()) {
-                context.trace.record(INDEXED_LVALUE_GET, expression, results.getResultingCall());
-                return DataFlowUtils.checkType(results.getResultingDescriptor().getReturnType(), expression, contextWithExpectedType);
-            }
-        }
-        context.trace.report(NO_GET_METHOD.on(expression.getIndicesNode()));
-        return null;
+    public JetType visitArrayAccessExpression(JetArrayAccessExpression expression, ExpressionTypingContext context) {
+        return resolveArrayAccessGetMethod(expression, context.replaceExpectedType(NO_EXPECTED_TYPE));
     }
 
     @Nullable
     public JetType getTypeForBinaryCall(JetScope scope, String name, ExpressionTypingContext context, JetBinaryExpression binaryExpression) {
         ExpressionReceiver receiver = safeGetExpressionReceiver(facade, binaryExpression.getLeft(), context.replaceScope(scope));
-        OverloadResolutionResults<FunctionDescriptor> results = context.replaceScope(scope).resolveCallWithGivenNameToDescriptor(
+        return OverloadResolutionResultsUtil.getResultType(getResolutionResultsForBinaryCall(scope, name, context, binaryExpression, receiver));
+    }
+
+    @NotNull
+    /*package*/ OverloadResolutionResults<FunctionDescriptor> getResolutionResultsForBinaryCall(JetScope scope, String name, ExpressionTypingContext context, JetBinaryExpression binaryExpression, ExpressionReceiver receiver) {
+//        ExpressionReceiver receiver = safeGetExpressionReceiver(facade, binaryExpression.getLeft(), context.replaceScope(scope));
+        return context.replaceScope(scope).resolveCallWithGivenNameToDescriptor(
                 CallMaker.makeCall(receiver, binaryExpression),
                 binaryExpression.getOperationReference(),
                 name
         );
-        if (results.isSuccess()) {
-            return results.getResultingDescriptor().getReturnType();
-        }
-        return null;
     }
 
     @Override
@@ -999,45 +986,38 @@ public class BasicExpressionTypingVisitor extends ExpressionTypingVisitor {
         context.trace.report(UNSUPPORTED.on(element, getClass().getCanonicalName()));
         return null;
     }
-    
-    /*package*/ JetType resolveArrayAccessToLValue(@NotNull JetArrayAccessExpression arrayAccessExpression, @NotNull JetExpression right, @NotNull ExpressionTypingContext context, final boolean getterNeeded) {
-        ExpressionReceiver receiver = getExpressionReceiver(facade, arrayAccessExpression.getArrayExpression(), context.replaceExpectedType(NO_EXPECTED_TYPE));
-        if (receiver == null) return null;
 
-        OverloadResolutionResults<FunctionDescriptor> getFunctionResults = null;
-        if (getterNeeded) {
-            TemporaryBindingTrace temporaryTrace = TemporaryBindingTrace.create(context.trace);
-            getFunctionResults = context.replaceExpectedType(TypeUtils.NO_EXPECTED_TYPE).replaceBindingTrace(temporaryTrace).resolveCallWithGivenName(
-                    CallMaker.makeArrayGetCall(receiver, arrayAccessExpression),
-                    arrayAccessExpression,
-                    "get");
-            if (!getFunctionResults.isSuccess()) {
-                temporaryTrace.commit();
-                context.trace.report(NO_GET_METHOD.on(arrayAccessExpression.getIndicesNode()));
-                return null;
-            }
-            context.trace.record(INDEXED_LVALUE_GET, arrayAccessExpression, getFunctionResults.getResultingCall());
-        }
+    @Nullable
+    /*package*/ JetType resolveArrayAccessSetMethod(@NotNull JetArrayAccessExpression arrayAccessExpression, @NotNull JetExpression rightHandSide, @NotNull ExpressionTypingContext context, @NotNull BindingTrace traceForResolveResult) {
+        return resolveArrayAccessSpecialMethod(arrayAccessExpression, rightHandSide, context, traceForResolveResult, false);
+    }
 
-        Call call = CallMaker.makeArraySetCall(receiver, arrayAccessExpression, right);
-        OverloadResolutionResults<FunctionDescriptor> setFunctionResults = context.replaceExpectedType(TypeUtils.NO_EXPECTED_TYPE).resolveCallWithGivenName(
-                call,
+    @Nullable
+    /*package*/ JetType resolveArrayAccessGetMethod(@NotNull JetArrayAccessExpression arrayAccessExpression, @NotNull ExpressionTypingContext context) {
+        return resolveArrayAccessSpecialMethod(arrayAccessExpression, null, context, context.trace, true);
+    }
+
+    @Nullable
+    private JetType resolveArrayAccessSpecialMethod(@NotNull JetArrayAccessExpression arrayAccessExpression,
+                                                    @Nullable JetExpression rightHandSide, //only for 'set' method
+                                                    @NotNull ExpressionTypingContext context,
+                                                    @NotNull BindingTrace traceForResolveResult,
+                                                    boolean isGet) {
+        JetType arrayType = facade.getType(arrayAccessExpression.getArrayExpression(), context);
+        if (arrayType == null) return null;
+
+        ExpressionReceiver receiver = new ExpressionReceiver(arrayAccessExpression.getArrayExpression(), arrayType);
+        if (!isGet) assert rightHandSide != null;
+        OverloadResolutionResults<FunctionDescriptor> functionResults = context.resolveCallWithGivenName(
+                isGet ? CallMaker.makeArrayGetCall(receiver, arrayAccessExpression) : CallMaker.makeArraySetCall(receiver, arrayAccessExpression, rightHandSide),
                 arrayAccessExpression,
-                "set");
-        if (!setFunctionResults.isSuccess()) {
-            context.trace.report(NO_SET_METHOD.on(arrayAccessExpression.getIndicesNode()));
-            if (getterNeeded) {
-                return getFunctionResults.getResultingDescriptor().getReturnType();
-            }
+                isGet ? "get" : "set");
+        if (!functionResults.isSuccess()) {
+            JetContainerNode indicesNode = arrayAccessExpression.getIndicesNode();
+            traceForResolveResult.report(isGet ? NO_GET_METHOD.on(indicesNode) : NO_SET_METHOD.on(indicesNode));
             return null;
         }
-        FunctionDescriptor setFunctionDescriptor = setFunctionResults.getResultingDescriptor();
-        context.trace.record(INDEXED_LVALUE_SET, arrayAccessExpression, setFunctionResults.getResultingCall());
-
-        if (getterNeeded) {
-            return getFunctionResults.getResultingDescriptor().getReturnType();
-        }
-
-        return setFunctionDescriptor.getReturnType();
+        traceForResolveResult.record(isGet ? INDEXED_LVALUE_GET : INDEXED_LVALUE_SET, arrayAccessExpression, functionResults.getResultingCall());
+        return functionResults.getResultingDescriptor().getReturnType();
     }
 }
\ No newline at end of file
diff --git a/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/ExpressionTypingVisitorForStatements.java b/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/ExpressionTypingVisitorForStatements.java
index d3551d4041a..715b697741d 100644
--- a/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/ExpressionTypingVisitorForStatements.java
+++ b/compiler/frontend/src/org/jetbrains/jet/lang/types/expressions/ExpressionTypingVisitorForStatements.java
@@ -1,23 +1,29 @@
 package org.jetbrains.jet.lang.types.expressions;
 
+import com.google.common.collect.Sets;
 import com.intellij.psi.tree.IElementType;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.jetbrains.jet.lang.descriptors.*;
 import org.jetbrains.jet.lang.diagnostics.Errors;
 import org.jetbrains.jet.lang.psi.*;
-import org.jetbrains.jet.lang.resolve.BindingContext;
-import org.jetbrains.jet.lang.resolve.DescriptorUtils;
-import org.jetbrains.jet.lang.resolve.TemporaryBindingTrace;
-import org.jetbrains.jet.lang.resolve.TopDownAnalyzer;
+import org.jetbrains.jet.lang.resolve.*;
+import org.jetbrains.jet.lang.resolve.calls.OverloadResolutionResults;
+import org.jetbrains.jet.lang.resolve.calls.OverloadResolutionResultsUtil;
+import org.jetbrains.jet.lang.resolve.calls.ResolvedCall;
 import org.jetbrains.jet.lang.resolve.scopes.JetScope;
 import org.jetbrains.jet.lang.resolve.scopes.WritableScope;
+import org.jetbrains.jet.lang.resolve.scopes.receivers.ExpressionReceiver;
 import org.jetbrains.jet.lang.types.JetStandardClasses;
 import org.jetbrains.jet.lang.types.JetType;
 import org.jetbrains.jet.lang.types.TypeUtils;
 import org.jetbrains.jet.lexer.JetTokens;
 
+import java.util.Collection;
+
 import static org.jetbrains.jet.lang.diagnostics.Errors.*;
+import static org.jetbrains.jet.lang.resolve.BindingContext.AMBIGUOUS_REFERENCE_TARGET;
+import static org.jetbrains.jet.lang.resolve.BindingContext.REFERENCE_TARGET;
 import static org.jetbrains.jet.lang.resolve.BindingContext.VARIABLE_REASSIGNMENT;
 
 /**
@@ -140,63 +146,74 @@ public class ExpressionTypingVisitorForStatements extends ExpressionTypingVisito
     }
 
     protected JetType visitAssignmentOperation(JetBinaryExpression expression, ExpressionTypingContext contextWithExpectedType) {
-        ExpressionTypingContext context = contextWithExpectedType.replaceExpectedType(TypeUtils.NO_EXPECTED_TYPE).replaceExpectedReturnType(TypeUtils.NO_EXPECTED_TYPE);
-        // If there's += (or similar op) defined as such, we just call it.
+        //There is a temporary binding trace for an opportunity to resolve set method for array if needed (the initial trace should be used there)
+        TemporaryBindingTrace temporaryBindingTrace = TemporaryBindingTrace.create(contextWithExpectedType.trace);
+        ExpressionTypingContext context = contextWithExpectedType.replaceExpectedType(TypeUtils.NO_EXPECTED_TYPE).replaceExpectedReturnType(TypeUtils.NO_EXPECTED_TYPE).replaceBindingTrace(temporaryBindingTrace);
+
         JetSimpleNameExpression operationSign = expression.getOperationReference();
         IElementType operationType = operationSign.getReferencedNameElementType();
-        String name = OperatorConventions.ASSIGNMENT_OPERATIONS.get(operationType);
-
-        TemporaryBindingTrace temporaryBindingTrace = TemporaryBindingTrace.create(context.trace);
-        JetType assignmentOperationType = basic.getTypeForBinaryCall(scope, name, context.replaceBindingTrace(temporaryBindingTrace), expression);
-
         JetExpression left = JetPsiUtil.deparenthesize(expression.getLeft());
-        // If there isn't, we call plus (or like) and then assign
-        if (assignmentOperationType == null) {
-            String counterpartName = OperatorConventions.BINARY_OPERATION_NAMES.get(OperatorConventions.ASSIGNMENT_OPERATION_COUNTERPARTS.get(operationType));
+        if (left == null) return null;
 
-            if (left instanceof JetArrayAccessExpression) {
-                JetArrayAccessExpression arrayAccessExpression = (JetArrayAccessExpression) left;
-                basic.resolveArrayAccessToLValue(arrayAccessExpression, expression.getRight(), context.replaceScope(scope), false);
-            }
-            assignmentOperationType = basic.getTypeForBinaryCall(scope, counterpartName, context, expression);
-            if (assignmentOperationType != null) {
-                context.trace.record(VARIABLE_REASSIGNMENT, expression);
-                ExpressionTypingUtils.checkWrappingInRef(expression.getLeft(), context);
-            }
-        }
-        else {
+        JetType leftType = facade.getType(left, context);
+        if (leftType == null) {
+            facade.getType(expression.getRight(), context);
+            context.trace.report(UNRESOLVED_REFERENCE.on(operationSign));
             temporaryBindingTrace.commit();
-            checkReassignment(expression, context, assignmentOperationType, left);
+            return null;
+        }
+        ExpressionReceiver receiver = new ExpressionReceiver(left, leftType);
+
+        // We check that defined only one of '+=' and '+' operations, and call it (in the case '+' we then also assign)
+        // Check for '+='
+        String name = OperatorConventions.ASSIGNMENT_OPERATIONS.get(operationType);
+        TemporaryBindingTrace assignmentOperationTrace = TemporaryBindingTrace.create(context.trace);
+        OverloadResolutionResults<FunctionDescriptor> assignmentOperationDescriptors = basic.getResolutionResultsForBinaryCall(scope, name, context.replaceBindingTrace(assignmentOperationTrace), expression, receiver);
+        JetType assignmentOperationType = OverloadResolutionResultsUtil.getResultType(assignmentOperationDescriptors);
+
+        // Check for '+'
+        String counterpartName = OperatorConventions.BINARY_OPERATION_NAMES.get(OperatorConventions.ASSIGNMENT_OPERATION_COUNTERPARTS.get(operationType));
+        TemporaryBindingTrace binaryOperationTrace = TemporaryBindingTrace.create(context.trace);
+        OverloadResolutionResults<FunctionDescriptor> binaryOperationDescriptors = basic.getResolutionResultsForBinaryCall(scope, counterpartName, context.replaceBindingTrace(binaryOperationTrace), expression, receiver);
+        JetType binaryOperationType = OverloadResolutionResultsUtil.getResultType(binaryOperationDescriptors);
+
+        JetType type = assignmentOperationType != null ? assignmentOperationType : binaryOperationType;
+        if (assignmentOperationType != null && binaryOperationType != null) {
+            OverloadResolutionResults<FunctionDescriptor> ambiguityResolutionResults = OverloadResolutionResultsUtil.ambiguity(assignmentOperationDescriptors, binaryOperationDescriptors);
+            context.trace.report(ASSIGN_OPERATOR_AMBIGUITY.on(operationSign, ambiguityResolutionResults.getResultingCalls()));
+            Collection<DeclarationDescriptor> descriptors = Sets.newHashSet();
+            for (ResolvedCall<? extends FunctionDescriptor> call : ambiguityResolutionResults.getResultingCalls()) {
+                descriptors.add(call.getResultingDescriptor());
+            }
+            facade.getType(expression.getRight(), context);
+            context.trace.record(AMBIGUOUS_REFERENCE_TARGET, operationSign, descriptors);
+        }
+        else if (assignmentOperationType != null) {
+            assignmentOperationTrace.commit();
+            if (!JetStandardClasses.isUnit(assignmentOperationType)) {
+                context.trace.report(ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT.on(operationSign, assignmentOperationDescriptors.getResultingDescriptor(), operationSign));
+            }
+        } else {
+            binaryOperationTrace.commit();
+            context.trace.record(VARIABLE_REASSIGNMENT, expression);
+            ExpressionTypingUtils.checkWrappingInRef(expression.getLeft(), context);
+            if (left instanceof JetArrayAccessExpression) {
+                ExpressionTypingContext contextForResolve = context.replaceScope(scope).replaceBindingTrace(TemporaryBindingTrace.create(contextWithExpectedType.trace));
+                basic.resolveArrayAccessSetMethod((JetArrayAccessExpression) left, expression.getRight(), contextForResolve, context.trace);
+            }
         }
         basic.checkLValue(context.trace, expression.getLeft());
-        return checkAssignmentType(assignmentOperationType, expression, contextWithExpectedType);
+        temporaryBindingTrace.commit();
+        return checkAssignmentType(type, expression, contextWithExpectedType);
     }
 
-    private void checkReassignment(@NotNull JetBinaryExpression expression, @NotNull ExpressionTypingContext context,
-                                   @NotNull JetType assignmentOperationType, @Nullable JetExpression left) {
-        if (left == null) return;
-        JetType leftType = facade.getType(left, context.replaceScope(scope));
-        if (leftType == null) return;
-        boolean isUnit = context.semanticServices.getTypeChecker().
-                isSubtypeOf(assignmentOperationType, JetStandardClasses.getUnitType());
-        if (isUnit) return;
-
-        if (context.semanticServices.getTypeChecker().isSubtypeOf(assignmentOperationType, leftType)) {
-            context.trace.record(VARIABLE_REASSIGNMENT, expression);
-        }
-        else {
-            context.trace.report(TYPE_MISMATCH.on(expression, leftType, assignmentOperationType));
-        }
-    }
-
-
     protected JetType visitAssignment(JetBinaryExpression expression, ExpressionTypingContext contextWithExpectedType) {
         ExpressionTypingContext context = contextWithExpectedType.replaceExpectedType(TypeUtils.NO_EXPECTED_TYPE).replaceExpectedReturnType(TypeUtils.NO_EXPECTED_TYPE);
         JetExpression left = JetPsiUtil.deparenthesize(expression.getLeft());
         JetExpression right = expression.getRight();
         if (left instanceof JetArrayAccessExpression) {
             JetArrayAccessExpression arrayAccessExpression = (JetArrayAccessExpression) left;
-            JetType assignmentType = basic.resolveArrayAccessToLValue(arrayAccessExpression, right, context.replaceScope(scope), false); //todo
+            JetType assignmentType = basic.resolveArrayAccessSetMethod(arrayAccessExpression, right, context.replaceScope(scope), context.trace);
             basic.checkLValue(context.trace, arrayAccessExpression);
             return checkAssignmentType(assignmentType, expression, contextWithExpectedType);
         }
diff --git a/compiler/testData/codegen/classes/overloadPlusAssignReturn.jet b/compiler/testData/codegen/classes/overloadPlusAssignReturn.jet
index 42671b9f138..c5236b0ace4 100644
--- a/compiler/testData/codegen/classes/overloadPlusAssignReturn.jet
+++ b/compiler/testData/codegen/classes/overloadPlusAssignReturn.jet
@@ -7,7 +7,7 @@ class ArrayWrapper<T>() {
         contents.add(item)
     }
 
-    fun plusAssign(rhs: ArrayWrapper<T>): ArrayWrapper<T> {
+    fun plus(rhs: ArrayWrapper<T>): ArrayWrapper<T> {
         val result = ArrayWrapper<T>()
         result.contents.addAll(contents)
         result.contents.addAll(rhs.contents)
diff --git a/compiler/testData/codegen/regressions/kt471.kt b/compiler/testData/codegen/regressions/kt471.kt
index 0c33b4fe0e5..bc452506e78 100644
--- a/compiler/testData/codegen/regressions/kt471.kt
+++ b/compiler/testData/codegen/regressions/kt471.kt
@@ -37,7 +37,7 @@ fun test4() : Boolean {
     return true
 }
 
-/*fun test5() : Boolean {
+fun test5() : Boolean {
     var mnr  = Array<MyNumber>(2,{MyNumber(42)})
     mnr[0]++
     if (mnr[0].i  != 43) return false
@@ -69,7 +69,7 @@ fun test8() : Boolean {
     if (mnr[1].i  != 42) return false
     return true
 }
-*/
+
 
 fun box() : String {
     var m  = MyNumber(42)
@@ -78,7 +78,7 @@ fun box() : String {
     if (!test2()) return "fail test 2"
     if (!test3()) return "fail test 3"
     if (!test4()) return "fail test 4"
-/*
+
     if (!test5()) return "fail test 5"
     if (!test6()) return "fail test 6"
     if (!test7()) return "fail test 7"
@@ -86,11 +86,11 @@ fun box() : String {
 
 
     ++m
-    if (m.i != 44) return "fail 0"
+    if (m.i != 43) return "fail 0"
 
     var m1 = ++m
-    if (m1.i != 46) return "fail 3"
-    if (m.i  != 46) return "fail 4"
-*/
+    if (m1.i != 44) return "fail 3"
+    if (m.i  != 44) return "fail 4"
+
     return "OK"
 }
diff --git a/compiler/testData/diagnostics/tests/IncDec.jet b/compiler/testData/diagnostics/tests/IncDec.jet
index f0cab587ec0..36f235c0734 100644
--- a/compiler/testData/diagnostics/tests/IncDec.jet
+++ b/compiler/testData/diagnostics/tests/IncDec.jet
@@ -35,12 +35,12 @@ class UnitIncDec() {
 
 fun testUnitIncDec() {
   var x = UnitIncDec()
-  x++
-  ++x
-  x--
-  --x
-  x = <!TYPE_MISMATCH, UNUSED_CHANGED_VALUE!>x++<!>
-  x = <!TYPE_MISMATCH, UNUSED_CHANGED_VALUE!>x--<!>
-  x = <!TYPE_MISMATCH!>++x<!>
-  x = <!TYPE_MISMATCH, UNUSED_VALUE!>--x<!>
+  x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>
+  <!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>x
+  x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!>
+  <!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!>x
+  x = <!UNUSED_CHANGED_VALUE!>x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!><!>
+  x = <!UNUSED_CHANGED_VALUE!>x<!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!><!>
+  x = <!INC_DEC_SHOULD_NOT_RETURN_UNIT!>++<!>x
+  x = <!UNUSED_VALUE!><!INC_DEC_SHOULD_NOT_RETURN_UNIT!>--<!>x<!>
 }
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/tests/regressions/kt618.jet b/compiler/testData/diagnostics/tests/regressions/kt618.jet
index 2ef9ef9d760..616f0283f8f 100644
--- a/compiler/testData/diagnostics/tests/regressions/kt618.jet
+++ b/compiler/testData/diagnostics/tests/regressions/kt618.jet
@@ -20,9 +20,9 @@ class B() {
 
 fun main(args : Array<String>) {
     var c = B()
-    <!TYPE_MISMATCH!>c += B()<!>
-    <!TYPE_MISMATCH!>c *= B()<!>
-    <!TYPE_MISMATCH!>c /= B()<!>
-    <!TYPE_MISMATCH!>c -= B()<!>
-    <!TYPE_MISMATCH!>c %= B()<!>
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>+=<!> B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>*=<!> B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>/=<!> B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>-=<!> B()
+    c <!ASSIGNMENT_OPERATOR_SHOULD_RETURN_UNIT!>%=<!> B()
 }
\ No newline at end of file
diff --git a/idea/testData/checker/IncDec.jet b/idea/testData/checker/IncDec.jet
index aa46004ffc2..b43ade29028 100644
--- a/idea/testData/checker/IncDec.jet
+++ b/idea/testData/checker/IncDec.jet
@@ -35,12 +35,12 @@ class UnitIncDec() {
 
 fun testUnitIncDec() {
   var x = UnitIncDec()
-  x++
-  ++x
-  x--
-  --x
-  x = <error>x++</error>
-  x = <error>x--</error>
-  x = <error>++x</error>
-  x = <error>--x</error>
+  x<error>++</error>
+  <error>++</error>x
+  x<error>--</error>
+  <error>--</error>x
+  x = <warning>x<error>++</error></warning>
+  x = <warning>x<error>--</error></warning>
+  x = <error>++</error>x
+  x = <warning><error>--</error>x</warning>
 }
\ No newline at end of file
diff --git a/stdlib/ktSrc/String.kt b/stdlib/ktSrc/String.kt
index df38013e610..e756e338ff4 100644
--- a/stdlib/ktSrc/String.kt
+++ b/stdlib/ktSrc/String.kt
@@ -2,8 +2,6 @@ package std
 
 import java.io.StringReader
 
-inline fun <T> T?.plus(str: String?) : String { return toString() + str }
-
 inline fun String.lastIndexOf(s: String)  = (this as java.lang.String).lastIndexOf(s)
 
 inline fun String.lastIndexOf(s: Char) = (this as java.lang.String).lastIndexOf(s.toString())