source: branches/3119_AdditionalShapeConstraintFeatures/HeuristicLab.ExtLibs/HeuristicLab.NRefactory/5.5.0/NRefactory.CSharp-5.5.0/Analysis/NullValueAnalysis.cs

// 
// NullValueAnalysis.cs
//  
// Author:
//       Luís Reis <luiscubal@gmail.com>
// 
// Copyright (c) 2013 Luís Reis
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Text;
using ICSharpCode.NRefactory.CSharp.Resolver;
using ICSharpCode.NRefactory.Semantics;
using ICSharpCode.NRefactory.TypeSystem;
using ICSharpCode.NRefactory.CSharp.Refactoring;
using ICSharpCode.NRefactory.PatternMatching;
using ICSharpCode.NRefactory.CSharp;
using ICSharpCode.NRefactory.Utils;

namespace ICSharpCode.NRefactory.CSharp.Analysis
{
  public class NullValueAnalysis
  {
    sealed class VariableStatusInfo : IEquatable<VariableStatusInfo>, IEnumerable<KeyValuePair<string, NullValueStatus>>
    {
      readonly Dictionary<string, NullValueStatus> VariableStatus = new Dictionary<string, NullValueStatus>();

      public NullValueStatus this[string name]
      {
        get {
          NullValueStatus status;
          if (VariableStatus.TryGetValue(name, out status)) {
            return status;
          }
          return NullValueStatus.UnreachableOrInexistent;
        }
        set {
          if (value == NullValueStatus.UnreachableOrInexistent) {
            VariableStatus.Remove(name);
          } else {
            VariableStatus [name] = value;
          }
        }
      }

      /// <summary>
      /// Modifies the variable state to consider a new incoming path
      /// </summary>
      /// <returns><c>true</c>, if the state has changed, <c>false</c> otherwise.</returns>
      /// <param name="incomingState">The variable state of the incoming path</param>
      public bool ReceiveIncoming(VariableStatusInfo incomingState)
      {
        bool changed = false;
        var listOfVariables = VariableStatus.Keys.Concat(incomingState.VariableStatus.Keys).ToList();
        foreach (string variable in listOfVariables)
        {
          var newValue = CombineStatus(this [variable], incomingState [variable]);
          if (this [variable] != newValue) {
            this [variable] = newValue;
            changed = true;
          }
        }

        return changed;
      }

      public static NullValueStatus CombineStatus(NullValueStatus oldValue, NullValueStatus incomingValue)
      {
        if (oldValue == NullValueStatus.Error || incomingValue == NullValueStatus.Error)
          return NullValueStatus.Error;

        if (oldValue == NullValueStatus.UnreachableOrInexistent ||
            oldValue == NullValueStatus.Unassigned)
          return incomingValue;

        if (incomingValue == NullValueStatus.Unassigned) {
          return NullValueStatus.Unassigned;
        }

        if (oldValue == NullValueStatus.CapturedUnknown || incomingValue == NullValueStatus.CapturedUnknown) {
          //TODO: Check if this is right
          return NullValueStatus.CapturedUnknown;
        }

        if (oldValue == NullValueStatus.Unknown) {
          return NullValueStatus.Unknown;
        }

        if (oldValue == NullValueStatus.DefinitelyNull) {
          return incomingValue == NullValueStatus.DefinitelyNull ?
            NullValueStatus.DefinitelyNull : NullValueStatus.PotentiallyNull;
        }

        if (oldValue == NullValueStatus.DefinitelyNotNull) {
          if (incomingValue == NullValueStatus.Unknown)
            return NullValueStatus.Unknown;
          if (incomingValue == NullValueStatus.DefinitelyNotNull)
            return NullValueStatus.DefinitelyNotNull;
          return NullValueStatus.PotentiallyNull;
        }

        Debug.Assert(oldValue == NullValueStatus.PotentiallyNull);
        return NullValueStatus.PotentiallyNull;
      }

      public bool HasVariable(string variable) {
        return VariableStatus.ContainsKey(variable);
      }

      public VariableStatusInfo Clone() {
        var clone = new VariableStatusInfo();
        foreach (var item in VariableStatus) {
          clone.VariableStatus.Add(item.Key, item.Value);
        }
        return clone;
      }

      public override bool Equals(object obj)
      {
        return Equals(obj as VariableStatusInfo);
      }

      public bool Equals(VariableStatusInfo obj)
      {
        if (obj == null) {
          return false;
        }

        if (VariableStatus.Count != obj.VariableStatus.Count)
          return false;

        return VariableStatus.All(item => item.Value == obj[item.Key]);
      }

      public override int GetHashCode()
      {
        //STUB
        return VariableStatus.Count.GetHashCode();
      }

      public static bool operator ==(VariableStatusInfo obj1, VariableStatusInfo obj2) {
        return object.ReferenceEquals(obj1, null) ?
          object.ReferenceEquals(obj2, null) : obj1.Equals(obj2);
      }

      public static bool operator !=(VariableStatusInfo obj1, VariableStatusInfo obj2) {
        return !(obj1 == obj2);
      }

      public IEnumerator<KeyValuePair<string, NullValueStatus>> GetEnumerator()
      {
        return VariableStatus.GetEnumerator();
      }

      IEnumerator IEnumerable.GetEnumerator()
      {
        return GetEnumerator();
      }

      public override string ToString()
      {
        var builder = new StringBuilder("[");
        foreach (var item in this) {
          builder.Append(item.Key);
          builder.Append("=");
          builder.Append(item.Value);
        }
        builder.Append("]");
        return builder.ToString();
      }
    }

    sealed class NullAnalysisNode : ControlFlowNode
    {
      public readonly VariableStatusInfo VariableState = new VariableStatusInfo();
      public bool Visited { get; private set; }

      public NullAnalysisNode(Statement previousStatement, Statement nextStatement, ControlFlowNodeType type)
        : base(previousStatement, nextStatement, type)
      {
      }

      public bool ReceiveIncoming(VariableStatusInfo incomingState)
      {
        bool changed = VariableState.ReceiveIncoming(incomingState);
        if (!Visited) {
          Visited = true;
          return true;
        }
        return changed;
      }
    }

    sealed class NullAnalysisGraphBuilder : ControlFlowGraphBuilder
    {
      protected override ControlFlowNode CreateNode(Statement previousStatement, Statement nextStatement, ControlFlowNodeType type)
      {
        return new NullAnalysisNode(previousStatement, nextStatement, type);
      }
    }

    class PendingNode : IEquatable<PendingNode> {
      internal readonly NullAnalysisNode nodeToVisit;
      internal readonly VariableStatusInfo statusInfo;
      internal readonly ComparableList<NullAnalysisNode> pendingTryFinallyNodes;
      internal readonly NullAnalysisNode nodeAfterFinally;

      internal PendingNode(NullAnalysisNode nodeToVisit, VariableStatusInfo statusInfo)
        : this(nodeToVisit, statusInfo, new ComparableList<NullAnalysisNode>(), null)
      {
      }

      public PendingNode(NullAnalysisNode nodeToVisit, VariableStatusInfo statusInfo, ComparableList<NullAnalysisNode> pendingFinallyNodes, NullAnalysisNode nodeAfterFinally)
      {
        this.nodeToVisit = nodeToVisit;
        this.statusInfo = statusInfo;
        this.pendingTryFinallyNodes = pendingFinallyNodes;
        this.nodeAfterFinally = nodeAfterFinally;
      }

      public override bool Equals(object obj)
      {
        return Equals(obj as PendingNode);
      }

      public bool Equals(PendingNode obj) {
        if (obj == null) return false;

        if (nodeToVisit != obj.nodeToVisit) return false;
        if (statusInfo != obj.statusInfo) return false;
        if (pendingTryFinallyNodes != obj.pendingTryFinallyNodes) return false;
        if (nodeAfterFinally != obj.nodeAfterFinally) return false;

        return true;
      }

      public override int GetHashCode()
      {
        return nodeToVisit.GetHashCode() ^
          statusInfo.GetHashCode() ^
          pendingTryFinallyNodes.GetHashCode() ^
          (nodeAfterFinally == null ? 0 : nodeAfterFinally.GetHashCode());
      }
    }

    readonly BaseRefactoringContext context;
    readonly NullAnalysisVisitor visitor;
    List<NullAnalysisNode> allNodes;
    readonly HashSet<PendingNode> nodesToVisit = new HashSet<PendingNode>();
    Dictionary<Statement, NullAnalysisNode> nodeBeforeStatementDict;
    Dictionary<Statement, NullAnalysisNode> nodeAfterStatementDict;
    readonly Dictionary<Expression, NullValueStatus> expressionResult = new Dictionary<Expression, NullValueStatus>();

    public NullValueAnalysis(BaseRefactoringContext context, MethodDeclaration methodDeclaration, CancellationToken cancellationToken)
      : this(context, methodDeclaration.Body, methodDeclaration.Parameters, cancellationToken)
    {
    }

    readonly IEnumerable<ParameterDeclaration> parameters;
    readonly Statement rootStatement;

    readonly CancellationToken cancellationToken;

    public NullValueAnalysis(BaseRefactoringContext context, Statement rootStatement, IEnumerable<ParameterDeclaration> parameters, CancellationToken cancellationToken)
    {
      if (rootStatement == null)
        throw new ArgumentNullException("rootStatement");
      if (context == null)
        throw new ArgumentNullException("context");

      this.context = context;
      this.rootStatement = rootStatement;
      this.parameters = parameters;
      this.visitor = new NullAnalysisVisitor(this);
      this.cancellationToken = cancellationToken;
    }

    /// <summary>
    /// Sets the local variable value.
    /// This method does not change anything if identifier does not refer to a local variable.
    /// Do not use this in variable declarations since resolving the variable won't work yet.
    /// </summary>
    /// <returns><c>true</c>, if local variable value was set, <c>false</c> otherwise.</returns>
    /// <param name="data">The variable status data to change.</param>
    /// <param name="identifierNode">The identifier to set.</param>
    /// <param name="identifierName">The name of the identifier to set.</param>
    /// <param name="value">The value to set the identifier.</param>
    bool SetLocalVariableValue (VariableStatusInfo data, AstNode identifierNode, string identifierName, NullValueStatus value) {
      var resolveResult = context.Resolve(identifierNode);
      if (resolveResult is LocalResolveResult) {
        if (data [identifierName] != NullValueStatus.CapturedUnknown) {
          data [identifierName] = value;

          return true;
        }
      }
      return false;
    }

    bool SetLocalVariableValue (VariableStatusInfo data, IdentifierExpression identifierExpression, NullValueStatus value) {
      return SetLocalVariableValue(data, identifierExpression, identifierExpression.Identifier, value);
    }

    bool SetLocalVariableValue (VariableStatusInfo data, Identifier identifier, NullValueStatus value) {
      return SetLocalVariableValue(data, identifier, identifier.Name, value);
    }

    void SetupNode(NullAnalysisNode node)
    {
      foreach (var parameter in parameters) {
        var resolveResult = context.Resolve(parameter.Type);
        node.VariableState[parameter.Name] = GetInitialVariableStatus(resolveResult);
      }

      nodesToVisit.Add(new PendingNode(node, node.VariableState));
    }

    static bool IsTypeNullable(IType type)
    {
      return type.IsReferenceType == true || type.FullName == "System.Nullable";
    }

    public bool IsParametersAreUninitialized {
      get;
      set;
    }

    NullValueStatus GetInitialVariableStatus(ResolveResult resolveResult)
    {
      var typeResolveResult = resolveResult as TypeResolveResult;
      if (typeResolveResult == null) {
        return NullValueStatus.Error;
      }
      var type = typeResolveResult.Type;
      if (type.IsReferenceType == null) {
        return NullValueStatus.Error;
      }
      if (!IsParametersAreUninitialized)
        return NullValueStatus.DefinitelyNotNull;
      return IsTypeNullable(type) ? NullValueStatus.PotentiallyNull : NullValueStatus.DefinitelyNotNull;
    }

    public void Analyze()
    {
      var cfgBuilder = new NullAnalysisGraphBuilder();
      allNodes = cfgBuilder.BuildControlFlowGraph(rootStatement, cancellationToken).Cast<NullAnalysisNode>().ToList();
      nodeBeforeStatementDict = allNodes.Where(node => node.Type == ControlFlowNodeType.StartNode || node.Type == ControlFlowNodeType.BetweenStatements)
        .ToDictionary(node => node.NextStatement);
      nodeAfterStatementDict = allNodes.Where(node => node.Type == ControlFlowNodeType.BetweenStatements || node.Type == ControlFlowNodeType.EndNode)
        .ToDictionary(node => node.PreviousStatement);

      foreach (var node in allNodes) {
        if (node.Type == ControlFlowNodeType.StartNode && node.NextStatement == rootStatement) {
          Debug.Assert(!nodesToVisit.Any());

          SetupNode(node);
        }
      }

      while (nodesToVisit.Any()) {
        var nodeToVisit = nodesToVisit.First();
        nodesToVisit.Remove(nodeToVisit);

        Visit(nodeToVisit);
      }
    }

    int visits = 0;

    public int NodeVisits
    {
      get {
        return visits;
      }
    }

    void Visit(PendingNode nodeInfo)
    {
      cancellationToken.ThrowIfCancellationRequested();

      var node = nodeInfo.nodeToVisit;
      var statusInfo = nodeInfo.statusInfo;

      visits++;
      if (visits > 100) {
        //Visiting way too often, let's enter fast mode
        //Fast mode is slighly less accurate but visits each node less times
        nodesToVisit.RemoveWhere(candidate => candidate.nodeToVisit == nodeInfo.nodeToVisit &&
                                 candidate.pendingTryFinallyNodes.Equals(nodeInfo.pendingTryFinallyNodes) &&
                                 candidate.nodeAfterFinally == nodeInfo.nodeAfterFinally);
        statusInfo = node.VariableState;
      }

      var nextStatement = node.NextStatement;
      VariableStatusInfo outgoingStatusInfo = statusInfo;
      VisitorResult result = null;

      if (nextStatement != null && (!(nextStatement is DoWhileStatement) || node.Type == ControlFlowNodeType.LoopCondition)) {
        result = nextStatement.AcceptVisitor(visitor, statusInfo);
        if (result == null) {
          Console.WriteLine("Failure in {0}", nextStatement);
          throw new InvalidOperationException();
        }

        outgoingStatusInfo = result.Variables;
      }

      if ((result == null || !result.ThrowsException) && node.Outgoing.Any()) {
        var tryFinallyStatement = nextStatement as TryCatchStatement;

        foreach (var outgoingEdge in node.Outgoing) {
          VariableStatusInfo edgeInfo;
          edgeInfo = outgoingStatusInfo.Clone();

          if (node.Type == ControlFlowNodeType.EndNode) {
            var previousBlock = node.PreviousStatement as BlockStatement;
            if (previousBlock != null) {
              //We're leaving a block statement.
              //As such, we'll remove the variables that were declared *in* the loop
              //This helps GetVariableStatusAfter/BeforeStatement be more accurate
              //and prevents some redundant revisiting.

              foreach (var variableInitializer in previousBlock.Statements
                       .OfType<VariableDeclarationStatement>()
                       .SelectMany(declaration => declaration.Variables)) {

                edgeInfo [variableInitializer.Name] = NullValueStatus.UnreachableOrInexistent;
              }
            }
          }

          if (tryFinallyStatement != null) {
            //With the exception of try statements, this needs special handling:
            //we'll set all changed variables to Unknown or CapturedUnknown
            if (outgoingEdge.To.NextStatement == tryFinallyStatement.FinallyBlock) {
              foreach (var identifierExpression in tryFinallyStatement.TryBlock.Descendants.OfType<IdentifierExpression>()) {
                //TODO: Investigate CaptureUnknown
                SetLocalVariableValue(edgeInfo, identifierExpression, NullValueStatus.Unknown);
              }
            } else {
              var clause = tryFinallyStatement.CatchClauses
                .FirstOrDefault(candidateClause => candidateClause.Body == outgoingEdge.To.NextStatement);

              if (clause != null) {
                SetLocalVariableValue(edgeInfo, clause.VariableNameToken, NullValueStatus.DefinitelyNotNull);

                foreach (var identifierExpression in tryFinallyStatement.TryBlock.Descendants.OfType<IdentifierExpression>()) {
                  //TODO: Investigate CaptureUnknown
                  SetLocalVariableValue(edgeInfo, identifierExpression, NullValueStatus.Unknown);
                }
              }
            }
          }

          if (result != null) {
            switch (outgoingEdge.Type) {
              case ControlFlowEdgeType.ConditionTrue:
                if (result.KnownBoolResult == false) {
                  //No need to explore this path -- expression is known to be false
                  continue;
                }
                edgeInfo = result.TruePathVariables;
                break;
              case ControlFlowEdgeType.ConditionFalse:
                if (result.KnownBoolResult == true) {
                  //No need to explore this path -- expression is known to be true
                  continue;
                }
                edgeInfo = result.FalsePathVariables;
                break;
            }
          }

          if (outgoingEdge.IsLeavingTryFinally) {
            var nodeAfterFinally = (NullAnalysisNode)outgoingEdge.To;
            var finallyNodes = outgoingEdge.TryFinallyStatements.Select(tryFinally => nodeBeforeStatementDict [tryFinally.FinallyBlock]).ToList();
            var nextNode = finallyNodes.First();
            var remainingFinallyNodes = new ComparableList<NullAnalysisNode>(finallyNodes.Skip(1));
            //We have to visit the node even if ReceiveIncoming returns false
            //since the finallyNodes/nodeAfterFinally might be different even if the values of variables are the same -- and they need to be visited either way!
            //TODO 1: Is there any point in visiting the finally statement here?
            //TODO 2: Do we need the ReceiveIncoming at all?
            nextNode.ReceiveIncoming(edgeInfo);
            nodesToVisit.Add(new PendingNode(nextNode, edgeInfo, remainingFinallyNodes, nodeAfterFinally));
          } else {
            var outgoingNode = (NullAnalysisNode)outgoingEdge.To;
            if (outgoingNode.ReceiveIncoming(edgeInfo)) {
              nodesToVisit.Add(new PendingNode(outgoingNode, edgeInfo));
            }
          }
        }
      } else {
        //We found a return/throw/yield break or some other termination node
        var finallyBlockStarts = nodeInfo.pendingTryFinallyNodes;
        var nodeAfterFinally = nodeInfo.nodeAfterFinally;

        if (finallyBlockStarts.Any()) {
          var nextNode = finallyBlockStarts.First();
          if (nextNode.ReceiveIncoming(outgoingStatusInfo))
            nodesToVisit.Add(new PendingNode(nextNode, outgoingStatusInfo, new ComparableList<NullAnalysisNode>(finallyBlockStarts.Skip(1)), nodeInfo.nodeAfterFinally));
        } else if (nodeAfterFinally != null && nodeAfterFinally.ReceiveIncoming(outgoingStatusInfo)) {
          nodesToVisit.Add(new PendingNode(nodeAfterFinally, outgoingStatusInfo));
        } else {
          //Maybe we finished a try/catch/finally statement the "normal" way (no direct jumps)
          //so let's check that case
          var statement = node.PreviousStatement ?? node.NextStatement;
          Debug.Assert(statement != null);
          var parent = statement.GetParent<Statement>();
          var parentTryCatch = parent as TryCatchStatement;
          if (parentTryCatch != null) {
            var nextNode = nodeAfterStatementDict [parentTryCatch];
            if (nextNode.ReceiveIncoming(outgoingStatusInfo)) {
              nodesToVisit.Add(new PendingNode(nextNode, outgoingStatusInfo));
            }
          }
        }
      }
    }

    public NullValueStatus GetExpressionResult(Expression expr)
    {
      if (expr == null)
        throw new ArgumentNullException("expr");

      NullValueStatus info;
      if (expressionResult.TryGetValue(expr, out info)) {
        return info;
      }

      return NullValueStatus.UnreachableOrInexistent;
    }

    public NullValueStatus GetVariableStatusBeforeStatement(Statement stmt, string variableName)
    {
      if (stmt == null)
        throw new ArgumentNullException("stmt");
      if (variableName == null)
        throw new ArgumentNullException("variableName");

      NullAnalysisNode node;
      if (nodeBeforeStatementDict.TryGetValue(stmt, out node)) {
        return node.VariableState [variableName];
      }

      return NullValueStatus.UnreachableOrInexistent;
    }

    public NullValueStatus GetVariableStatusAfterStatement(Statement stmt, string variableName)
    {
      if (stmt == null)
        throw new ArgumentNullException("stmt");
      if (variableName == null)
        throw new ArgumentNullException("variableName");

      NullAnalysisNode node;
      if (nodeAfterStatementDict.TryGetValue(stmt, out node)) {
        return node.VariableState [variableName];
      }

      return NullValueStatus.UnreachableOrInexistent;
    }

    class ConditionalBranchInfo
    {
      /// <summary>
      /// True if the variable is null for the true path, false if it is false for the true path.
      /// </summary>
      public Dictionary<string, bool> TrueResultVariableNullStates = new Dictionary<string, bool>();
      /// <summary>
      /// True if the variable is null for the false path, false if it is false for the false path.
      /// </summary>
      public Dictionary<string, bool> FalseResultVariableNullStates = new Dictionary<string, bool>();
    }

    class VisitorResult
    {
      /// <summary>
      /// Indicates the return value of the expression.
      /// </summary>
      /// <remarks>
      /// Only applicable for expressions.
      /// </remarks>
      public NullValueStatus NullableReturnResult;

      /// <summary>
      /// Indicates the value of each item in an array or linq query.
      /// </summary>
      public NullValueStatus EnumeratedValueResult;

      /// <summary>
      /// Information that indicates the restrictions to add
      /// when branching.
      /// </summary>
      /// <remarks>
      /// Used in if/else statements, conditional expressions and
      /// while statements.
      /// </remarks>
      public ConditionalBranchInfo ConditionalBranchInfo;

      /// <summary>
      /// The state of the variables after the expression is executed.
      /// </summary>
      public VariableStatusInfo Variables;

      /// <summary>
      /// The expression is known to be invalid and trigger an error
      /// (e.g. a NullReferenceException)
      /// </summary>
      public bool ThrowsException;

      /// <summary>
      /// The known bool result of an expression.
      /// </summary>
      public bool? KnownBoolResult;

      public static VisitorResult ForEnumeratedValue(VariableStatusInfo variables, NullValueStatus itemValues)
      {
        var result = new VisitorResult();
        result.NullableReturnResult = NullValueStatus.DefinitelyNotNull;
        result.EnumeratedValueResult = itemValues;
        result.Variables = variables.Clone();
        return result;
      }

      public static VisitorResult ForValue(VariableStatusInfo variables, NullValueStatus returnValue)
      {
        var result = new VisitorResult();
        result.NullableReturnResult = returnValue;
        result.Variables = variables.Clone();
        return result;
      }

      public static VisitorResult ForBoolValue(VariableStatusInfo variables, bool newValue)
      {
        var result = new VisitorResult();
        result.NullableReturnResult = NullValueStatus.DefinitelyNotNull; //Bool expressions are never null
        result.KnownBoolResult = newValue;
        result.Variables = variables.Clone();
        return result;
      }

      public static VisitorResult ForException(VariableStatusInfo variables) {
        var result = new VisitorResult();
        result.NullableReturnResult = NullValueStatus.UnreachableOrInexistent;
        result.ThrowsException = true;
        result.Variables = variables.Clone();
        return result;
      }

      public VisitorResult Negated {
        get {
          var result = new VisitorResult();
          if (NullableReturnResult.IsDefiniteValue()) {
            result.NullableReturnResult = NullableReturnResult == NullValueStatus.DefinitelyNull
              ? NullValueStatus.DefinitelyNotNull : NullValueStatus.DefinitelyNull;
          } else {
            result.NullableReturnResult = NullableReturnResult;
          }
          result.Variables = Variables.Clone();
          result.KnownBoolResult = !KnownBoolResult;
          if (ConditionalBranchInfo != null) {
            result.ConditionalBranchInfo = new ConditionalBranchInfo();
            foreach (var item in ConditionalBranchInfo.TrueResultVariableNullStates) {
              result.ConditionalBranchInfo.FalseResultVariableNullStates [item.Key] = item.Value;
            }
            foreach (var item in ConditionalBranchInfo.FalseResultVariableNullStates) {
              result.ConditionalBranchInfo.TrueResultVariableNullStates [item.Key] = item.Value;
            }
          }
          return result;
        }
      }

      public VariableStatusInfo TruePathVariables {
        get {
          var variables = Variables.Clone();
          if (ConditionalBranchInfo != null) {
            foreach (var item in ConditionalBranchInfo.TrueResultVariableNullStates) {
              variables [item.Key] = item.Value ? NullValueStatus.DefinitelyNull : NullValueStatus.DefinitelyNotNull;
            }
          }
          return variables;
        }
      }

      public VariableStatusInfo FalsePathVariables {
        get {
          var variables = Variables.Clone();
          if (ConditionalBranchInfo != null) {
            foreach (var item in ConditionalBranchInfo.FalseResultVariableNullStates) {
              variables [item.Key] = item.Value ? NullValueStatus.DefinitelyNull : NullValueStatus.DefinitelyNotNull;
            }
          }
          return variables;
        }
      }

      public static VisitorResult AndOperation(VisitorResult tentativeLeftResult, VisitorResult tentativeRightResult)
      {
        var result = new VisitorResult();
        result.KnownBoolResult = tentativeLeftResult.KnownBoolResult & tentativeRightResult.KnownBoolResult;

        var trueTruePath = tentativeRightResult.TruePathVariables;
        var trueFalsePath = tentativeRightResult.FalsePathVariables;
        var falsePath = tentativeLeftResult.FalsePathVariables;

        var trueVariables = trueTruePath;

        var falseVariables = trueFalsePath.Clone();
        falseVariables.ReceiveIncoming(falsePath);
        result.Variables = trueVariables.Clone();
        result.Variables.ReceiveIncoming(falseVariables);

        result.ConditionalBranchInfo = new ConditionalBranchInfo();

        foreach (var variable in trueVariables) {
          if (!variable.Value.IsDefiniteValue())
            continue;

          string variableName = variable.Key;

          if (variable.Value != result.Variables[variableName]) {
            bool isNull = variable.Value == NullValueStatus.DefinitelyNull;
            result.ConditionalBranchInfo.TrueResultVariableNullStates.Add(variableName, isNull);
          }
        }

        foreach (var variable in falseVariables) {
          if (!variable.Value.IsDefiniteValue())
            continue;

          string variableName = variable.Key;

          if (variable.Value != result.Variables [variableName]) {
            bool isNull = variable.Value == NullValueStatus.DefinitelyNull;
            result.ConditionalBranchInfo.FalseResultVariableNullStates.Add(variableName, isNull);
          }
        }

        return result;
      }

      public static VisitorResult OrOperation(VisitorResult tentativeLeftResult, VisitorResult tentativeRightResult)
      {
        return VisitorResult.AndOperation(tentativeLeftResult.Negated, tentativeRightResult.Negated).Negated;
      }
    }

    class NullAnalysisVisitor : DepthFirstAstVisitor<VariableStatusInfo, VisitorResult>
    {
      NullValueAnalysis analysis;

      public NullAnalysisVisitor(NullValueAnalysis analysis) {
        this.analysis = analysis;
      }

      protected override VisitorResult VisitChildren(AstNode node, VariableStatusInfo data)
      {
        Debug.Fail("Missing override for " + node.GetType().Name);
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }
      
      public override VisitorResult VisitNullNode(AstNode nullNode, VariableStatusInfo data)
      {
        // can occur due to syntax errors
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }
      
      public override VisitorResult VisitEmptyStatement(EmptyStatement emptyStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitBlockStatement(BlockStatement blockStatement, VariableStatusInfo data)
      {
        //We'll visit the child statements later (we'll visit each one directly from the CFG)
        //As such this is mostly a dummy node.
        return new VisitorResult { Variables = data };
      }

      public override VisitorResult VisitVariableDeclarationStatement(VariableDeclarationStatement variableDeclarationStatement, VariableStatusInfo data)
      {
        foreach (var variable in variableDeclarationStatement.Variables) {
          var result = variable.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return result;
          data = result.Variables;
        }

        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitVariableInitializer(VariableInitializer variableInitializer, VariableStatusInfo data)
      {
        if (variableInitializer.Initializer.IsNull) {
          data = data.Clone();
          data[variableInitializer.Name] = NullValueStatus.Unassigned;
        } else {
          var result = variableInitializer.Initializer.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return result;
          data = result.Variables.Clone();
          data[variableInitializer.Name] = result.NullableReturnResult;
        }

        return VisitorResult.ForValue(data, data [variableInitializer.Name]);
      }

      public override VisitorResult VisitIfElseStatement(IfElseStatement ifElseStatement, VariableStatusInfo data)
      {
        //We'll visit the true/false statements later (directly from the CFG)
        return ifElseStatement.Condition.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitWhileStatement(WhileStatement whileStatement, VariableStatusInfo data)
      {
        return whileStatement.Condition.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitDoWhileStatement(DoWhileStatement doWhileStatement, VariableStatusInfo data)
      {
        return doWhileStatement.Condition.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitForStatement(ForStatement forStatement, VariableStatusInfo data)
      {
        //The initializers, the embedded statement and the iterators aren't visited here
        //because they have their own CFG nodes.
        if (forStatement.Condition.IsNull)
          return VisitorResult.ForValue(data, NullValueStatus.Unknown);
        return forStatement.Condition.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitForeachStatement(ForeachStatement foreachStatement, VariableStatusInfo data)
      {
        var newVariable = foreachStatement.VariableNameToken;
        var inExpressionResult = foreachStatement.InExpression.AcceptVisitor(this, data);
        if (inExpressionResult.ThrowsException)
          return inExpressionResult;

        var newData = inExpressionResult.Variables.Clone();

        var resolveResult = analysis.context.Resolve(foreachStatement.VariableNameToken) as LocalResolveResult;
        if (resolveResult != null) {
          //C# 5.0 changed the meaning of foreach so that each iteration declares a new variable
          //as such, the variable is "uncaptured" only for C# >= 5.0
          if (analysis.context.Supports(new Version(5, 0)) || data[newVariable.Name] != NullValueStatus.CapturedUnknown) {
            newData[newVariable.Name] = NullValueAnalysis.IsTypeNullable(resolveResult.Type) ? inExpressionResult.EnumeratedValueResult : NullValueStatus.DefinitelyNotNull;
          }
        }

        return VisitorResult.ForValue(newData, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitUsingStatement(UsingStatement usingStatement, VariableStatusInfo data)
      {
        return usingStatement.ResourceAcquisition.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitFixedStatement(FixedStatement fixedStatement, VariableStatusInfo data)
      {
        foreach (var variable in fixedStatement.Variables) {
          var result = variable.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return result;
          data = result.Variables;
        }

        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitSwitchStatement(SwitchStatement switchStatement, VariableStatusInfo data)
      {
        //We could do better than this, but it would require special handling outside the visitor
        //so for now, for simplicity, we'll just take the easy way

        var tentativeResult = switchStatement.Expression.AcceptVisitor(this, data);
        if (tentativeResult.ThrowsException) {
          return tentativeResult;
        }

        foreach (var section in switchStatement.SwitchSections) {
          //No need to check for ThrowsException, since it will always be false (see VisitSwitchSection)
          section.AcceptVisitor(this, tentativeResult.Variables);
        }

        return VisitorResult.ForValue(tentativeResult.Variables, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitSwitchSection(SwitchSection switchSection, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitExpressionStatement(ExpressionStatement expressionStatement, VariableStatusInfo data)
      {
        return expressionStatement.Expression.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitReturnStatement(ReturnStatement returnStatement, VariableStatusInfo data)
      {
        if (returnStatement.Expression.IsNull)
          return VisitorResult.ForValue(data, NullValueStatus.Unknown);
        return returnStatement.Expression.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitTryCatchStatement(TryCatchStatement tryCatchStatement, VariableStatusInfo data)
      {
        //The needs special treatment in the analyser itself
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitBreakStatement(BreakStatement breakStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitContinueStatement(ContinueStatement continueStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitGotoStatement(GotoStatement gotoStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitGotoCaseStatement(GotoCaseStatement gotoCaseStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitGotoDefaultStatement(GotoDefaultStatement gotoDefaultStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitLabelStatement(LabelStatement labelStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitUnsafeStatement(UnsafeStatement unsafeStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitLockStatement(LockStatement lockStatement, VariableStatusInfo data)
      {
        var expressionResult = lockStatement.Expression.AcceptVisitor(this, data);
        if (expressionResult.ThrowsException)
          return expressionResult;

        if (expressionResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
          return VisitorResult.ForException(expressionResult.Variables);
        }

        var identifier = CSharpUtil.GetInnerMostExpression(lockStatement.Expression) as IdentifierExpression;
        if (identifier != null) {
          var identifierValue = expressionResult.Variables [identifier.Identifier];
          if (identifierValue != NullValueStatus.CapturedUnknown) {
            var newVariables = expressionResult.Variables.Clone();
            analysis.SetLocalVariableValue(newVariables, identifier, NullValueStatus.DefinitelyNotNull);

            return VisitorResult.ForValue(newVariables, NullValueStatus.Unknown);
          }
        }

        return VisitorResult.ForValue(expressionResult.Variables, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitThrowStatement(ThrowStatement throwStatement, VariableStatusInfo data)
      {
        if (throwStatement.Expression.IsNull)
          return VisitorResult.ForValue(data, NullValueStatus.DefinitelyNotNull);
        return throwStatement.Expression.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitYieldBreakStatement(YieldBreakStatement yieldBreakStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitYieldReturnStatement(YieldReturnStatement yieldReturnStatement, VariableStatusInfo data)
      {
        return yieldReturnStatement.Expression.AcceptVisitor(this, data);
      }

      public override VisitorResult VisitCheckedStatement(CheckedStatement checkedStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitUncheckedStatement(UncheckedStatement uncheckedStatement, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      void RegisterExpressionResult(Expression expression, NullValueStatus expressionResult)
      {
        NullValueStatus oldStatus;
        if (analysis.expressionResult.TryGetValue(expression, out oldStatus)) {
          analysis.expressionResult[expression] = VariableStatusInfo.CombineStatus(analysis.expressionResult[expression], expressionResult);
        }
        else {
          analysis.expressionResult[expression] = expressionResult;
        }
      }

      VisitorResult HandleExpressionResult(Expression expression, VariableStatusInfo dataAfterExpression, NullValueStatus expressionResult) {
        RegisterExpressionResult(expression, expressionResult);

        return VisitorResult.ForValue(dataAfterExpression, expressionResult);
      }

      VisitorResult HandleExpressionResult(Expression expression, VariableStatusInfo dataAfterExpression, bool expressionResult) {
        RegisterExpressionResult(expression, NullValueStatus.DefinitelyNotNull);

        return VisitorResult.ForBoolValue(dataAfterExpression, expressionResult);
      }

      VisitorResult HandleExpressionResult(Expression expression, VisitorResult result) {
        RegisterExpressionResult(expression, result.NullableReturnResult);

        return result;
      }

      public override VisitorResult VisitAssignmentExpression(AssignmentExpression assignmentExpression, VariableStatusInfo data)
      {
        var tentativeResult = assignmentExpression.Left.AcceptVisitor(this, data);
        if (tentativeResult.ThrowsException)
          return HandleExpressionResult(assignmentExpression, tentativeResult);
        tentativeResult = assignmentExpression.Right.AcceptVisitor(this, tentativeResult.Variables);
        if (tentativeResult.ThrowsException)
          return HandleExpressionResult(assignmentExpression, tentativeResult);

        var leftIdentifier = assignmentExpression.Left as IdentifierExpression;
        if (leftIdentifier != null) {
          var resolveResult = analysis.context.Resolve(leftIdentifier);
          if (resolveResult.IsError) {
            return HandleExpressionResult(assignmentExpression, data, NullValueStatus.Error);
          }

          if (resolveResult is LocalResolveResult) {
            var result = new VisitorResult();
            result.NullableReturnResult = tentativeResult.NullableReturnResult;
            result.Variables = tentativeResult.Variables.Clone();
            var oldValue = result.Variables [leftIdentifier.Identifier];

            if (assignmentExpression.Operator == AssignmentOperatorType.Assign ||
                oldValue == NullValueStatus.Unassigned ||
                oldValue == NullValueStatus.DefinitelyNotNull ||
                tentativeResult.NullableReturnResult == NullValueStatus.Error ||
                tentativeResult.NullableReturnResult == NullValueStatus.Unknown) {
              analysis.SetLocalVariableValue(result.Variables, leftIdentifier, tentativeResult.NullableReturnResult);
            } else {
              if (oldValue == NullValueStatus.DefinitelyNull) {
                //Do nothing --it'll remain null
              } else {
                analysis.SetLocalVariableValue(result.Variables, leftIdentifier, NullValueStatus.PotentiallyNull);
              }
            }

            return HandleExpressionResult(assignmentExpression, result);
          }
        }

        return HandleExpressionResult(assignmentExpression, tentativeResult);
      }

      public override VisitorResult VisitIdentifierExpression(IdentifierExpression identifierExpression, VariableStatusInfo data)
      {
        var resolveResult = analysis.context.Resolve(identifierExpression);
        if (resolveResult.IsError) {
          return HandleExpressionResult(identifierExpression, data, NullValueStatus.Error);
        }
        var local = resolveResult as LocalResolveResult;
        if (local != null) {
          var value = data [local.Variable.Name];
          if (value == NullValueStatus.CapturedUnknown)
            value = NullValueStatus.Unknown;
          return HandleExpressionResult(identifierExpression, data, value);
        }
        if (resolveResult.IsCompileTimeConstant) {
          object value = resolveResult.ConstantValue;
          if (value == null) {
            return HandleExpressionResult(identifierExpression, data, NullValueStatus.DefinitelyNull);
          }
          var boolValue = value as bool?;
          if (boolValue != null) {
            return VisitorResult.ForBoolValue(data, (bool)boolValue);
          }
          return HandleExpressionResult(identifierExpression, data, NullValueStatus.DefinitelyNotNull);
        }

        var memberResolveResult = resolveResult as MemberResolveResult;

        var returnValue = GetFieldReturnValue(memberResolveResult, data);

        return HandleExpressionResult(identifierExpression, data, returnValue);
      }

      public override VisitorResult VisitDefaultValueExpression(DefaultValueExpression defaultValueExpression, VariableStatusInfo data)
      {
        var resolveResult = analysis.context.Resolve(defaultValueExpression);
        if (resolveResult.IsError) {
          return HandleExpressionResult(defaultValueExpression, data, NullValueStatus.Unknown);
        }

        Debug.Assert(resolveResult.IsCompileTimeConstant);

        var status = resolveResult.ConstantValue == null  && resolveResult.Type.IsReferenceType != false ? NullValueStatus.DefinitelyNull : NullValueStatus.DefinitelyNotNull;
        return HandleExpressionResult(defaultValueExpression, data, status);
      }

      public override VisitorResult VisitNullReferenceExpression(NullReferenceExpression nullReferenceExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(nullReferenceExpression, data, NullValueStatus.DefinitelyNull);
      }

      public override VisitorResult VisitPrimitiveExpression(PrimitiveExpression primitiveExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(primitiveExpression, data, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitParenthesizedExpression(ParenthesizedExpression parenthesizedExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(parenthesizedExpression, parenthesizedExpression.Expression.AcceptVisitor(this, data));
      }

      public override VisitorResult VisitConditionalExpression(ConditionalExpression conditionalExpression, VariableStatusInfo data)
      {
        var tentativeBaseResult = conditionalExpression.Condition.AcceptVisitor(this, data);
        if (tentativeBaseResult.ThrowsException)
          return HandleExpressionResult(conditionalExpression, tentativeBaseResult);

        var conditionResolveResult = analysis.context.Resolve(conditionalExpression.Condition);

        if (tentativeBaseResult.KnownBoolResult == true || true.Equals(conditionResolveResult.ConstantValue)) {
          return HandleExpressionResult(conditionalExpression, conditionalExpression.TrueExpression.AcceptVisitor(this, tentativeBaseResult.TruePathVariables));
        }
        if (tentativeBaseResult.KnownBoolResult == false || false.Equals(conditionResolveResult.ConstantValue)) {
          return HandleExpressionResult(conditionalExpression, conditionalExpression.FalseExpression.AcceptVisitor(this, tentativeBaseResult.FalsePathVariables));
        }

        //No known bool result
        var trueCaseResult = conditionalExpression.TrueExpression.AcceptVisitor(this, tentativeBaseResult.TruePathVariables);
        if (trueCaseResult.ThrowsException) {
          //We know that the true case will never be completed, then the right case is the only possible route.
          return HandleExpressionResult(conditionalExpression, conditionalExpression.FalseExpression.AcceptVisitor(this, tentativeBaseResult.FalsePathVariables));
        }
        var falseCaseResult = conditionalExpression.FalseExpression.AcceptVisitor(this, tentativeBaseResult.FalsePathVariables);
        if (falseCaseResult.ThrowsException) {
          return HandleExpressionResult(conditionalExpression, trueCaseResult.Variables, true);
        }

        return HandleExpressionResult(conditionalExpression, VisitorResult.OrOperation(trueCaseResult, falseCaseResult));
      }

      public override VisitorResult VisitBinaryOperatorExpression(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data)
      {
        //Let's not evaluate the sides just yet because of ??, && and ||

        //We'll register the results here (with HandleExpressionResult)
        //so each Visit*Expression won't have to do it itself
        switch (binaryOperatorExpression.Operator) {
          case BinaryOperatorType.ConditionalAnd:
            return HandleExpressionResult(binaryOperatorExpression, VisitConditionalAndExpression(binaryOperatorExpression, data));
          case BinaryOperatorType.ConditionalOr:
            return HandleExpressionResult(binaryOperatorExpression, VisitConditionalOrExpression(binaryOperatorExpression, data));
          case BinaryOperatorType.NullCoalescing:
            return HandleExpressionResult(binaryOperatorExpression, VisitNullCoalescing(binaryOperatorExpression, data));
          case BinaryOperatorType.Equality:
            return HandleExpressionResult(binaryOperatorExpression, VisitEquality(binaryOperatorExpression, data));
          case BinaryOperatorType.InEquality:
            return HandleExpressionResult(binaryOperatorExpression, VisitEquality(binaryOperatorExpression, data).Negated);
          default:
            return HandleExpressionResult(binaryOperatorExpression, VisitOtherBinaryExpression(binaryOperatorExpression, data));
        }
      }

      VisitorResult VisitOtherBinaryExpression(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data)
      {
        var leftTentativeResult = binaryOperatorExpression.Left.AcceptVisitor(this, data);
        if (leftTentativeResult.ThrowsException)
          return leftTentativeResult;
        var rightTentativeResult = binaryOperatorExpression.Right.AcceptVisitor(this, leftTentativeResult.Variables);
        if (rightTentativeResult.ThrowsException)
          return rightTentativeResult;

        //TODO: Assuming operators are not overloaded by users
        // (or, if they are, that they retain similar behavior to the default ones)

        switch (binaryOperatorExpression.Operator) {
          case BinaryOperatorType.LessThan:
          case BinaryOperatorType.GreaterThan:
            //Operations < and > with nulls always return false
            //Those same operations will other values may or may not return false
            if (leftTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull &&
              rightTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
              return VisitorResult.ForBoolValue(rightTentativeResult.Variables, false);
            }
            //We don't know what the value is, but we know that both true and false are != null.
            return VisitorResult.ForValue(rightTentativeResult.Variables, NullValueStatus.DefinitelyNotNull);
          case BinaryOperatorType.LessThanOrEqual:
          case BinaryOperatorType.GreaterThanOrEqual:
            if (leftTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
              if (rightTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull)
                return VisitorResult.ForBoolValue(rightTentativeResult.Variables, true);
              if (rightTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNotNull)
                return VisitorResult.ForBoolValue(rightTentativeResult.Variables, false);
            } else if (leftTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNotNull) {
              if (rightTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull)
                return VisitorResult.ForBoolValue(rightTentativeResult.Variables, false);
            }

            return VisitorResult.ForValue(rightTentativeResult.Variables, NullValueStatus.Unknown);
          default:
            //Anything else: null + anything == anything + null == null.
            //not null + not null = not null
            if (leftTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
              return VisitorResult.ForValue(rightTentativeResult.Variables, NullValueStatus.DefinitelyNull);
            }
            if (leftTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNotNull) {
              if (rightTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull)
                return VisitorResult.ForValue(rightTentativeResult.Variables, NullValueStatus.DefinitelyNull);
              if (rightTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNotNull)
                return VisitorResult.ForValue(rightTentativeResult.Variables, NullValueStatus.DefinitelyNotNull);
            }

            return VisitorResult.ForValue(rightTentativeResult.Variables, NullValueStatus.Unknown);
        }
      }

      VisitorResult WithVariableValue(VisitorResult result, IdentifierExpression identifier, bool isNull)
      {
        var localVariableResult = analysis.context.Resolve(identifier) as LocalResolveResult;
        if (localVariableResult != null) {
          result.ConditionalBranchInfo.TrueResultVariableNullStates[identifier.Identifier] = isNull;
          if (isNull) {
            result.ConditionalBranchInfo.FalseResultVariableNullStates[identifier.Identifier] = false;
          }
        }
        return result;
      }

      VisitorResult VisitEquality(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data)
      {
        //TODO: Should this check for user operators?

        var tentativeLeftResult = binaryOperatorExpression.Left.AcceptVisitor(this, data);
        if (tentativeLeftResult.ThrowsException)
          return tentativeLeftResult;
        var tentativeRightResult = binaryOperatorExpression.Right.AcceptVisitor(this, tentativeLeftResult.Variables);
        if (tentativeRightResult.ThrowsException)
          return tentativeRightResult;

        if (tentativeLeftResult.KnownBoolResult != null && tentativeLeftResult.KnownBoolResult == tentativeRightResult.KnownBoolResult) {
          return VisitorResult.ForBoolValue(tentativeRightResult.Variables, true);
        }

        if (tentativeLeftResult.KnownBoolResult != null && tentativeLeftResult.KnownBoolResult == !tentativeRightResult.KnownBoolResult) {
          return VisitorResult.ForBoolValue(tentativeRightResult.Variables, false);
        }

        if (tentativeLeftResult.NullableReturnResult.IsDefiniteValue()) {
          if (tentativeRightResult.NullableReturnResult.IsDefiniteValue()) {
            if (tentativeLeftResult.NullableReturnResult == NullValueStatus.DefinitelyNull || tentativeRightResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
              return VisitorResult.ForBoolValue(tentativeRightResult.Variables, tentativeLeftResult.NullableReturnResult == tentativeRightResult.NullableReturnResult);
            }
          }
        }

        var result = new VisitorResult();
        result.Variables = tentativeRightResult.Variables;
        result.NullableReturnResult = NullValueStatus.Unknown;
        result.ConditionalBranchInfo = new ConditionalBranchInfo();

        if (tentativeRightResult.NullableReturnResult.IsDefiniteValue()) {
          var identifier = CSharpUtil.GetInnerMostExpression(binaryOperatorExpression.Left) as IdentifierExpression;

          if (identifier != null) {
            bool isNull = (tentativeRightResult.NullableReturnResult == NullValueStatus.DefinitelyNull);

            WithVariableValue(result, identifier, isNull);
          }
        }

        if (tentativeLeftResult.NullableReturnResult.IsDefiniteValue()) {
          var identifier = CSharpUtil.GetInnerMostExpression(binaryOperatorExpression.Right) as IdentifierExpression;

          if (identifier != null) {
            bool isNull = (tentativeLeftResult.NullableReturnResult == NullValueStatus.DefinitelyNull);

            WithVariableValue(result, identifier, isNull);
          }
        }

        return result;
      }

      VisitorResult VisitConditionalAndExpression(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data)
      {
        var tentativeLeftResult = binaryOperatorExpression.Left.AcceptVisitor(this, data);
        if (tentativeLeftResult.KnownBoolResult == false || tentativeLeftResult.ThrowsException) {
          return tentativeLeftResult;
        }

        var truePath = tentativeLeftResult.TruePathVariables;
        var tentativeRightResult = binaryOperatorExpression.Right.AcceptVisitor(this, truePath);
        if (tentativeRightResult.ThrowsException) {
          //If the true path throws an exception, then the only way for the expression to complete
          //successfully is if the left expression is false
          return VisitorResult.ForBoolValue(tentativeLeftResult.FalsePathVariables, false);
        }

        return VisitorResult.AndOperation(tentativeLeftResult, tentativeRightResult);
      }

      VisitorResult VisitConditionalOrExpression(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data)
      {
        var tentativeLeftResult = binaryOperatorExpression.Left.AcceptVisitor(this, data);
        if (tentativeLeftResult.KnownBoolResult == true || tentativeLeftResult.ThrowsException) {
          return tentativeLeftResult;
        }

        var falsePath = tentativeLeftResult.FalsePathVariables;
        var tentativeRightResult = binaryOperatorExpression.Right.AcceptVisitor(this, falsePath);
        if (tentativeRightResult.ThrowsException) {
          //If the false path throws an exception, then the only way for the expression to complete
          //successfully is if the left expression is true
          return VisitorResult.ForBoolValue(tentativeLeftResult.TruePathVariables, true);
        }

        return VisitorResult.OrOperation(tentativeLeftResult, tentativeRightResult);
      }

      VisitorResult VisitNullCoalescing(BinaryOperatorExpression binaryOperatorExpression, VariableStatusInfo data)
      {
        var leftTentativeResult = binaryOperatorExpression.Left.AcceptVisitor(this, data);
        if (leftTentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNotNull || leftTentativeResult.ThrowsException) {
          return leftTentativeResult;
        }

        //If the right side is found, then the left side is known to be null
        var newData = leftTentativeResult.Variables;
        var leftIdentifier = CSharpUtil.GetInnerMostExpression(binaryOperatorExpression.Left) as IdentifierExpression;
        if (leftIdentifier != null) {
          newData = newData.Clone();
          analysis.SetLocalVariableValue(newData, leftIdentifier, NullValueStatus.DefinitelyNull);
        }

        var rightTentativeResult = binaryOperatorExpression.Right.AcceptVisitor(this, newData);
        if (rightTentativeResult.ThrowsException) {
          //This means the left expression was not null all along (or else the expression will throw an exception)

          if (leftIdentifier != null) {
            newData = newData.Clone();
            analysis.SetLocalVariableValue(newData, leftIdentifier, NullValueStatus.DefinitelyNotNull);
            return VisitorResult.ForValue(newData, NullValueStatus.DefinitelyNotNull);
          }

          return VisitorResult.ForValue(leftTentativeResult.Variables, NullValueStatus.DefinitelyNotNull);
        }

        var mergedVariables = rightTentativeResult.Variables;
        var nullValue = rightTentativeResult.NullableReturnResult;

        if (leftTentativeResult.NullableReturnResult != NullValueStatus.DefinitelyNull) {
          mergedVariables = mergedVariables.Clone();
          mergedVariables.ReceiveIncoming(leftTentativeResult.Variables);
          if (nullValue == NullValueStatus.DefinitelyNull) {
            nullValue = NullValueStatus.PotentiallyNull;
          }
        }

        return VisitorResult.ForValue(mergedVariables, nullValue);
      }

      public override VisitorResult VisitUnaryOperatorExpression(UnaryOperatorExpression unaryOperatorExpression, VariableStatusInfo data)
      {
        //TODO: Again, what to do when overloaded operators are found?

        var tentativeResult = unaryOperatorExpression.Expression.AcceptVisitor(this, data);
        if (tentativeResult.ThrowsException)
          return HandleExpressionResult(unaryOperatorExpression, tentativeResult);

        if (unaryOperatorExpression.Operator == UnaryOperatorType.Not) {
          return HandleExpressionResult(unaryOperatorExpression, tentativeResult.Negated);
        }
        return HandleExpressionResult(unaryOperatorExpression, tentativeResult);
      }

      public override VisitorResult VisitInvocationExpression(InvocationExpression invocationExpression, VariableStatusInfo data)
      {
        //TODO: Handle some common methods such as string.IsNullOrEmpty

        var targetResult = invocationExpression.Target.AcceptVisitor(this, data);
        if (targetResult.ThrowsException)
          return HandleExpressionResult(invocationExpression, targetResult);

        data = targetResult.Variables;

        var methodResolveResult = analysis.context.Resolve(invocationExpression) as CSharpInvocationResolveResult;

        List<VisitorResult> parameterResults = new List<VisitorResult>();

        foreach (var argumentToHandle in invocationExpression.Arguments.Select((argument, parameterIndex) => new { argument, parameterIndex })) {
          var argument = argumentToHandle.argument;
          var parameterIndex = argumentToHandle.parameterIndex;

          var result = argument.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return HandleExpressionResult(invocationExpression, result);
          parameterResults.Add(result);

          var namedArgument = argument as NamedArgumentExpression;
          
          var directionExpression = (namedArgument == null ? argument : namedArgument.Expression) as DirectionExpression;
          if (directionExpression != null && methodResolveResult != null) {
            var identifier = directionExpression.Expression as IdentifierExpression;
            if (identifier != null) {
              //out and ref parameters do *NOT* capture the variable (since they must stop changing it by the time they return)
              var identifierResolveResult = analysis.context.Resolve(identifier) as LocalResolveResult;
              if (identifierResolveResult != null && IsTypeNullable(identifierResolveResult.Type)) {
                data = data.Clone();

                FixParameter(argument, methodResolveResult.Member.Parameters, parameterIndex, identifier, data);
              }
            }


            continue;
          }

          data = result.Variables;
        }

        var identifierExpression = CSharpUtil.GetInnerMostExpression(invocationExpression.Target) as IdentifierExpression;
        if (identifierExpression != null) {
          if (targetResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
            return HandleExpressionResult(invocationExpression, VisitorResult.ForException(data));
          }

          var descendentIdentifiers = invocationExpression.Arguments.SelectMany(argument => argument.DescendantsAndSelf).OfType<IdentifierExpression>();
          if (!descendentIdentifiers.Any(identifier => identifier.Identifier == identifierExpression.Identifier)) {
            //TODO: We can make this check better (see VisitIndexerExpression for more details)
            data = data.Clone();
            analysis.SetLocalVariableValue(data, identifierExpression, NullValueStatus.DefinitelyNotNull);
          }
        }

        return HandleExpressionResult(invocationExpression, GetMethodVisitorResult(methodResolveResult, data, parameterResults));
      }

      static VisitorResult GetMethodVisitorResult(CSharpInvocationResolveResult methodResolveResult, VariableStatusInfo data, List<VisitorResult> parameterResults)
      {
        if (methodResolveResult == null)
          return VisitorResult.ForValue(data, NullValueStatus.Unknown);

        var method = methodResolveResult.Member as IMethod;
        if (method != null) {
          if (method.GetAttribute(new FullTypeName(AnnotationNames.AssertionMethodAttribute)) != null) {
            var assertionParameters = method.Parameters.Select((parameter, index) => new { index, parameter })
              .Select(parameter => new { parameter.index, parameter.parameter, attributes = parameter.parameter.Attributes.Where(attribute => attribute.AttributeType.FullName == AnnotationNames.AssertionConditionAttribute).ToList() })
              .Where(parameter => parameter.attributes.Count() == 1)
              .Select(parameter => new { parameter.index, parameter.parameter, attribute = parameter.attributes[0] })
              .ToList();

            //Unclear what should be done if there are multiple assertion conditions
            if (assertionParameters.Count() == 1) {
              Debug.Assert(methodResolveResult.Arguments.Count == parameterResults.Count);

              var assertionParameter = assertionParameters [0];
              VisitorResult assertionParameterResult = null;

              object intendedResult = true;
              var positionalArgument = assertionParameter.attribute.PositionalArguments.FirstOrDefault() as MemberResolveResult;
              if (positionalArgument != null && positionalArgument.Type.FullName == AnnotationNames.AssertionConditionTypeAttribute) {
                switch (positionalArgument.Member.FullName) {
                  case AnnotationNames.AssertionConditionTypeIsTrue:
                    intendedResult = true;
                    break;
                  case AnnotationNames.AssertionConditionTypeIsFalse:
                    intendedResult = false;
                    break;
                  case AnnotationNames.AssertionConditionTypeIsNull:
                    intendedResult = null;
                    break;
                  case AnnotationNames.AssertionConditionTypeIsNotNull:
                    intendedResult = "<not-null>";
                    break;
                }
              }

              int parameterIndex = assertionParameter.index;
              if (assertionParameter.index < methodResolveResult.Arguments.Count && !(methodResolveResult.Arguments [assertionParameter.index] is NamedArgumentResolveResult)) {
                //Use index
                assertionParameterResult = parameterResults [assertionParameter.index];
              } else {
                //Use named argument
                int? nameIndex = methodResolveResult.Arguments.Select((argument, index) => new { argument, index})
                  .Where(argument => {
                    var namedArgument = argument.argument as NamedArgumentResolveResult;
                    return namedArgument != null && namedArgument.ParameterName == assertionParameter.parameter.Name;
                  }).Select(argument => (int?)argument.index).FirstOrDefault();

                if (nameIndex != null) {
                  parameterIndex = nameIndex.Value;
                  assertionParameterResult = parameterResults [nameIndex.Value];
                } else if (assertionParameter.parameter.IsOptional) {
                  //Try to use default value

                  if (intendedResult is string) {
                    if (assertionParameter.parameter.ConstantValue == null) {
                      return VisitorResult.ForException(data);
                    }
                  } else {
                    if (!object.Equals(assertionParameter.parameter.ConstantValue, intendedResult)) {
                      return VisitorResult.ForException(data);
                    }
                  }
                } else {
                  //The parameter was not specified, yet it is not optional?
                  return VisitorResult.ForException(data);
                }
              }

              //Now check assertion
              if (assertionParameterResult != null) {
                if (intendedResult is bool) {
                  if (assertionParameterResult.KnownBoolResult == !(bool)intendedResult) {
                    return VisitorResult.ForException(data);
                  }

                  data = (bool)intendedResult ? assertionParameterResult.TruePathVariables : assertionParameterResult.FalsePathVariables;
                } else {
                  bool shouldBeNull = intendedResult == null;

                  if (assertionParameterResult.NullableReturnResult == (shouldBeNull ? NullValueStatus.DefinitelyNotNull : NullValueStatus.DefinitelyNull)) {
                    return VisitorResult.ForException(data);
                  }

                  var parameterResolveResult = methodResolveResult.Arguments [parameterIndex];

                  LocalResolveResult localVariableResult = null;

                  var conversionResolveResult = parameterResolveResult as ConversionResolveResult;
                  if (conversionResolveResult != null) {
                    if (!IsTypeNullable(conversionResolveResult.Type)) {
                      if (intendedResult == null) {
                        return VisitorResult.ForException(data);
                      }
                    } else {
                      localVariableResult = conversionResolveResult.Input as LocalResolveResult;
                    }
                  } else {
                    localVariableResult = parameterResolveResult as LocalResolveResult;
                  }

                  if (localVariableResult != null && data[localVariableResult.Variable.Name] != NullValueStatus.CapturedUnknown) {
                    data = data.Clone();
                    data [localVariableResult.Variable.Name] = shouldBeNull ? NullValueStatus.DefinitelyNull : NullValueStatus.DefinitelyNotNull;
                  }
                }
              }
            }
          }
        }

        bool isNullable = IsTypeNullable(methodResolveResult.Type);
        if (!isNullable) {
          return VisitorResult.ForValue(data, NullValueStatus.DefinitelyNotNull);
        }

        if (method != null)
          return VisitorResult.ForValue(data, GetNullableStatus(method));

        return VisitorResult.ForValue(data, GetNullableStatus(methodResolveResult.TargetResult.Type.GetDefinition()));
      }

      static NullValueStatus GetNullableStatus(IEntity entity)
      {
        if (entity.DeclaringType != null && entity.DeclaringType.Kind == TypeKind.Delegate) {
          //Handle Delegate.Invoke method
          return GetNullableStatus(entity.DeclaringTypeDefinition);
        }
        
        return GetNullableStatus(fullTypeName => entity.GetAttribute(new FullTypeName(fullTypeName)));
      }

      static NullValueStatus GetNullableStatus(IParameter parameter)
      {
        return GetNullableStatus(fullTypeName => parameter.Attributes.FirstOrDefault(attribute => attribute.AttributeType.FullName == fullTypeName));
      }

      static NullValueStatus GetNullableStatus(Func<string, IAttribute> attributeGetter)
      {
        if (attributeGetter(AnnotationNames.NotNullAttribute) != null) {
          return NullValueStatus.DefinitelyNotNull;
        }
        if (attributeGetter(AnnotationNames.CanBeNullAttribute) != null) {
          return NullValueStatus.PotentiallyNull;
        }
        return NullValueStatus.Unknown;
      }

      public override VisitorResult VisitMemberReferenceExpression(MemberReferenceExpression memberReferenceExpression, VariableStatusInfo data)
      {
        var targetResult = memberReferenceExpression.Target.AcceptVisitor(this, data);
        if (targetResult.ThrowsException)
          return HandleExpressionResult(memberReferenceExpression, targetResult);

        var variables = targetResult.Variables;

        var memberResolveResult = analysis.context.Resolve(memberReferenceExpression) as MemberResolveResult;

        var targetIdentifier = CSharpUtil.GetInnerMostExpression(memberReferenceExpression.Target) as IdentifierExpression;
        if (targetIdentifier != null) {
          if (memberResolveResult == null) {
            var invocation = memberReferenceExpression.Parent as InvocationExpression;
            if (invocation != null) {
              memberResolveResult = analysis.context.Resolve(invocation) as MemberResolveResult;
            }
          }

          if (memberResolveResult != null && memberResolveResult.Member.FullName != "System.Nullable.HasValue") {
            var method = memberResolveResult.Member as IMethod;
            if (method == null || !method.IsExtensionMethod) {
              if (targetResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
                return HandleExpressionResult(memberReferenceExpression, VisitorResult.ForException(variables));
              }
              if (variables [targetIdentifier.Identifier] != NullValueStatus.CapturedUnknown) {
                variables = variables.Clone();
                analysis.SetLocalVariableValue(variables, targetIdentifier, NullValueStatus.DefinitelyNotNull);
              }
            }
          }
        }

        var returnValue = GetFieldReturnValue(memberResolveResult, data);
        return HandleExpressionResult(memberReferenceExpression, variables, returnValue);
      }

      static NullValueStatus GetFieldReturnValue(MemberResolveResult memberResolveResult, VariableStatusInfo data)
      {
        bool isNullable = memberResolveResult == null || IsTypeNullable(memberResolveResult.Type);
        if (!isNullable) {
          return NullValueStatus.DefinitelyNotNull;
        }

        if (memberResolveResult != null) {
          return GetNullableStatus(memberResolveResult.Member);
        }

        return NullValueStatus.Unknown;
      }

      public override VisitorResult VisitTypeReferenceExpression(TypeReferenceExpression typeReferenceExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(typeReferenceExpression, data, NullValueStatus.Unknown);

      }

      void FixParameter(Expression argument, IList<IParameter> parameters, int parameterIndex, IdentifierExpression identifier, VariableStatusInfo data)
      {
        NullValueStatus newValue = NullValueStatus.Unknown;
        if (argument is NamedArgumentExpression) {
          var namedResolveResult = analysis.context.Resolve(argument) as NamedArgumentResolveResult;
          if (namedResolveResult != null) {
            newValue = GetNullableStatus(namedResolveResult.Parameter);
          }
        }
        else {
          var parameter = parameters[parameterIndex];
          newValue = GetNullableStatus(parameter);
        }
        analysis.SetLocalVariableValue(data, identifier, newValue);
      }

      public override VisitorResult VisitObjectCreateExpression(ObjectCreateExpression objectCreateExpression, VariableStatusInfo data)
      {
        foreach (var argumentToHandle in objectCreateExpression.Arguments.Select((argument, parameterIndex) => new { argument, parameterIndex })) {
          var argument = argumentToHandle.argument;
          var parameterIndex = argumentToHandle.parameterIndex;

          var namedArgument = argument as NamedArgumentExpression;

          var directionExpression = (namedArgument == null ? argument : namedArgument.Expression) as DirectionExpression;
          if (directionExpression != null) {
            var identifier = directionExpression.Expression as IdentifierExpression;
            if (identifier != null && data [identifier.Identifier] != NullValueStatus.CapturedUnknown) {
              //out and ref parameters do *NOT* capture the variable (since they must stop changing it by the time they return)
              data = data.Clone();

              var constructorResolveResult = analysis.context.Resolve(objectCreateExpression) as CSharpInvocationResolveResult;
              if (constructorResolveResult != null)
                FixParameter(argument, constructorResolveResult.Member.Parameters, parameterIndex, identifier, data);
            }
            continue;
          }

          var argumentResult = argument.AcceptVisitor(this, data);
          if (argumentResult.ThrowsException)
            return argumentResult;

          data = argumentResult.Variables;
        }

        //Constructors never return null
        return HandleExpressionResult(objectCreateExpression, data, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitArrayCreateExpression(ArrayCreateExpression arrayCreateExpression, VariableStatusInfo data)
      {
        foreach (var argument in arrayCreateExpression.Arguments) {
          var result = argument.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return result;
          data = result.Variables.Clone();
        }

        if (arrayCreateExpression.Initializer.IsNull) {
          return HandleExpressionResult(arrayCreateExpression, data, NullValueStatus.DefinitelyNotNull);
        }

        return HandleExpressionResult(arrayCreateExpression, arrayCreateExpression.Initializer.AcceptVisitor(this, data));
      }

      public override VisitorResult VisitArrayInitializerExpression(ArrayInitializerExpression arrayInitializerExpression, VariableStatusInfo data)
      {
        if (arrayInitializerExpression.IsSingleElement) {
          return HandleExpressionResult(arrayInitializerExpression, arrayInitializerExpression.Elements.Single().AcceptVisitor(this, data));
        }
        if (!arrayInitializerExpression.Elements.Any()) {
          //Empty array
          return HandleExpressionResult(arrayInitializerExpression, VisitorResult.ForValue(data, NullValueStatus.Unknown));
        }

        NullValueStatus enumeratedValue = NullValueStatus.UnreachableOrInexistent;
        foreach (var element in arrayInitializerExpression.Elements) {
          var result = element.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return result;
          data = result.Variables.Clone();
          enumeratedValue = VariableStatusInfo.CombineStatus(enumeratedValue, result.NullableReturnResult);

        }
        return HandleExpressionResult(arrayInitializerExpression, VisitorResult.ForEnumeratedValue(data, enumeratedValue));
      }

      public override VisitorResult VisitAnonymousTypeCreateExpression(AnonymousTypeCreateExpression anonymousTypeCreateExpression, VariableStatusInfo data)
      {
        foreach (var initializer in anonymousTypeCreateExpression.Initializers) {
          var result = initializer.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return result;
          data = result.Variables;
        }

        return HandleExpressionResult(anonymousTypeCreateExpression, data, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitLambdaExpression(LambdaExpression lambdaExpression, VariableStatusInfo data)
      {
        var newData = data.Clone();

        var identifiers = lambdaExpression.Descendants.OfType<IdentifierExpression>();
        foreach (var identifier in identifiers) {
          //Check if it is in a "change-null-state" context
          //For instance, x++ does not change the null state
          //but `x = y` does.
          if (identifier.Parent is AssignmentExpression && identifier.Role == AssignmentExpression.LeftRole) {
            var parent = (AssignmentExpression)identifier.Parent;
            if (parent.Operator != AssignmentOperatorType.Assign) {
              continue;
            }
          } else {
            //No other context matters
            //Captured variables are never passed by reference (out/ref)
            continue;
          }

          //At this point, we know there's a good chance the variable has been changed
          var identifierResolveResult = analysis.context.Resolve(identifier) as LocalResolveResult;
          if (identifierResolveResult != null && IsTypeNullable(identifierResolveResult.Type)) {
            analysis.SetLocalVariableValue(newData, identifier, NullValueStatus.CapturedUnknown);
          }
        }

        //The lambda itself is known not to be null
        return HandleExpressionResult(lambdaExpression, newData, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitAnonymousMethodExpression(AnonymousMethodExpression anonymousMethodExpression, VariableStatusInfo data)
      {
        var newData = data.Clone();

        var identifiers = anonymousMethodExpression.Descendants.OfType<IdentifierExpression>();
        foreach (var identifier in identifiers) {
          //Check if it is in a "change-null-state" context
          //For instance, x++ does not change the null state
          //but `x = y` does.
          if (identifier.Parent is AssignmentExpression && identifier.Role == AssignmentExpression.LeftRole) {
            var parent = (AssignmentExpression)identifier.Parent;
            if (parent.Operator != AssignmentOperatorType.Assign) {
              continue;
            }
          } else {
            //No other context matters
            //Captured variables are never passed by reference (out/ref)
            continue;
          }

          //At this point, we know there's a good chance the variable has been changed
          var identifierResolveResult = analysis.context.Resolve(identifier) as LocalResolveResult;
          if (identifierResolveResult != null && IsTypeNullable(identifierResolveResult.Type)) {
            analysis.SetLocalVariableValue(newData, identifier, NullValueStatus.CapturedUnknown);
          }
        }

        //The anonymous method itself is known not to be null
        return HandleExpressionResult(anonymousMethodExpression, newData, NullValueStatus.DefinitelyNotNull);
      }


      public override VisitorResult VisitNamedExpression(NamedExpression namedExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(namedExpression, namedExpression.Expression.AcceptVisitor(this, data));
      }

      public override VisitorResult VisitAsExpression(AsExpression asExpression, VariableStatusInfo data)
      {
        var tentativeResult = asExpression.Expression.AcceptVisitor(this, data);
        if (tentativeResult.ThrowsException)
          return tentativeResult;

        NullValueStatus result;
        if (tentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
          result = NullValueStatus.DefinitelyNull;
        } else {
          var asResolveResult = analysis.context.Resolve(asExpression) as CastResolveResult;
          if (asResolveResult == null ||
              asResolveResult.IsError ||
              asResolveResult.Input.Type.Kind == TypeKind.Unknown ||
              asResolveResult.Type.Kind == TypeKind.Unknown) {

            result = NullValueStatus.Unknown;
          } else {
            var conversion = new CSharpConversions(analysis.context.Compilation);
            var foundConversion = conversion.ExplicitConversion(asResolveResult.Input.Type, asResolveResult.Type);

            if (foundConversion == Conversion.None) {
              result = NullValueStatus.DefinitelyNull;
            } else if (foundConversion == Conversion.IdentityConversion) {
              result = tentativeResult.NullableReturnResult;
            } else {
              result = NullValueStatus.PotentiallyNull;
            }
          }
        }
        return HandleExpressionResult(asExpression, tentativeResult.Variables, result);
      }

      public override VisitorResult VisitCastExpression(CastExpression castExpression, VariableStatusInfo data)
      {
        var tentativeResult = castExpression.Expression.AcceptVisitor(this, data);
        if (tentativeResult.ThrowsException)
          return tentativeResult;

        NullValueStatus result;
        if (tentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull) {
          result = NullValueStatus.DefinitelyNull;
        } else {
          result = NullValueStatus.Unknown;
        }

        VariableStatusInfo variables = tentativeResult.Variables;

        var resolveResult = analysis.context.Resolve(castExpression) as CastResolveResult;
        if (resolveResult != null && !IsTypeNullable(resolveResult.Type)) {
          if (result == NullValueStatus.DefinitelyNull) {
            return HandleExpressionResult(castExpression, VisitorResult.ForException(tentativeResult.Variables));
          }

          var identifierExpression = CSharpUtil.GetInnerMostExpression(castExpression.Expression) as IdentifierExpression;
          if (identifierExpression != null) {
            var currentValue = variables [identifierExpression.Identifier];
            if (currentValue != NullValueStatus.CapturedUnknown &&
                currentValue != NullValueStatus.UnreachableOrInexistent &&
                currentValue != NullValueStatus.DefinitelyNotNull) {
              //DefinitelyNotNull is included in this list because if that's the status
              // then we don't need to change anything

              variables = variables.Clone();
              variables [identifierExpression.Identifier] = NullValueStatus.DefinitelyNotNull;
            }
          }

          result = NullValueStatus.DefinitelyNotNull;
        }

        return HandleExpressionResult(castExpression, variables, result);
      }

      public override VisitorResult VisitIsExpression(IsExpression isExpression, VariableStatusInfo data)
      {
        var tentativeResult = isExpression.Expression.AcceptVisitor(this, data);
        if (tentativeResult.ThrowsException)
          return tentativeResult;

        //TODO: Consider, for instance: new X() is X. The result is known to be true, so we can use KnownBoolValue
        return HandleExpressionResult(isExpression, tentativeResult.Variables, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitDirectionExpression(DirectionExpression directionExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(directionExpression, directionExpression.Expression.AcceptVisitor(this, data));
      }

      public override VisitorResult VisitCheckedExpression(CheckedExpression checkedExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(checkedExpression, checkedExpression.Expression.AcceptVisitor(this, data));
      }

      public override VisitorResult VisitUncheckedExpression(UncheckedExpression uncheckedExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(uncheckedExpression, uncheckedExpression.Expression.AcceptVisitor(this, data));
      }

      public override VisitorResult VisitThisReferenceExpression(ThisReferenceExpression thisReferenceExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(thisReferenceExpression, data, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitIndexerExpression(IndexerExpression indexerExpression, VariableStatusInfo data)
      {
        var tentativeResult = indexerExpression.Target.AcceptVisitor(this, data);
        if (tentativeResult.ThrowsException)
          return tentativeResult;

        data = tentativeResult.Variables;

        foreach (var argument in indexerExpression.Arguments) {
          var result = argument.AcceptVisitor(this, data);
          if (result.ThrowsException)
            return result;
          data = result.Variables.Clone();
        }

        IdentifierExpression targetAsIdentifier = CSharpUtil.GetInnerMostExpression(indexerExpression.Target) as IdentifierExpression;
        if (targetAsIdentifier != null) {
          if (tentativeResult.NullableReturnResult == NullValueStatus.DefinitelyNull)
            return HandleExpressionResult(indexerExpression, VisitorResult.ForException(data));

          //If this doesn't cause an exception, then the target is not null
          //But we won't set it if it has been changed
          var descendentIdentifiers = indexerExpression.Arguments
            .SelectMany(argument => argument.DescendantsAndSelf).OfType<IdentifierExpression>();
          if (!descendentIdentifiers.Any(identifier => identifier.Identifier == targetAsIdentifier.Identifier)) {
            //TODO: this check might be improved to include more legitimate cases
            //A good check will necessarily have to consider captured variables
            data = data.Clone();
            analysis.SetLocalVariableValue(data, targetAsIdentifier, NullValueStatus.DefinitelyNotNull);
          }
        }

        var indexerResolveResult = analysis.context.Resolve(indexerExpression) as CSharpInvocationResolveResult;
        bool isNullable = indexerResolveResult == null || IsTypeNullable(indexerResolveResult.Type);
        
        var returnValue = isNullable ? NullValueStatus.Unknown : NullValueStatus.DefinitelyNotNull;
        return HandleExpressionResult(indexerExpression, data, returnValue);
      }

      public override VisitorResult VisitBaseReferenceExpression(BaseReferenceExpression baseReferenceExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(baseReferenceExpression, data, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitTypeOfExpression(TypeOfExpression typeOfExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(typeOfExpression, data, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitSizeOfExpression(SizeOfExpression sizeOfExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(sizeOfExpression, data, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitPointerReferenceExpression(PointerReferenceExpression pointerReferenceExpression, VariableStatusInfo data)
      {
        var targetResult = pointerReferenceExpression.Target.AcceptVisitor(this, data);
        if (targetResult.ThrowsException)
          return targetResult;
        return HandleExpressionResult(pointerReferenceExpression, targetResult.Variables, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitStackAllocExpression(StackAllocExpression stackAllocExpression, VariableStatusInfo data)
      {
        var countResult = stackAllocExpression.CountExpression.AcceptVisitor(this, data);
        if (countResult.ThrowsException)
          return countResult;
        return HandleExpressionResult(stackAllocExpression, countResult.Variables, NullValueStatus.DefinitelyNotNull);
      }

      public override VisitorResult VisitNamedArgumentExpression(NamedArgumentExpression namedArgumentExpression, VariableStatusInfo data)
      {
        return HandleExpressionResult(namedArgumentExpression, namedArgumentExpression.Expression.AcceptVisitor(this, data));
      }

      public override VisitorResult VisitUndocumentedExpression(UndocumentedExpression undocumentedExpression, VariableStatusInfo data)
      {
        throw new NotImplementedException();
      }

      public override VisitorResult VisitQueryExpression(QueryExpression queryExpression, VariableStatusInfo data)
      {
        VariableStatusInfo outgoingData = data.Clone();
        NullValueStatus? outgoingEnumeratedValue = null;
        var clauses = queryExpression.Clauses.ToList();

        var backtracingClauses = (from item in clauses.Select((clause, i) => new { clause, i })
                                        where item.clause is QueryFromClause || item.clause is QueryJoinClause || item.clause is QueryContinuationClause
                                        select item.i).ToList();

        var beforeClauseVariableStates = Enumerable.Range(0, clauses.Count).ToDictionary(clauseIndex => clauseIndex,
                                                                                         clauseIndex => new VariableStatusInfo());
        var afterClauseVariableStates = Enumerable.Range(0, clauses.Count).ToDictionary(clauseIndex => clauseIndex,
                                                                                        clauseIndex => new VariableStatusInfo());

        VisitorResult lastValidResult = null;
        int currentClauseIndex = 0;
        for (;;) {
          VisitorResult result = null;
          QueryClause clause = null;
          bool backtrack = false;

          if (currentClauseIndex >= clauses.Count) {
            backtrack = true;
          } else {
            clause = clauses [currentClauseIndex];
            beforeClauseVariableStates [currentClauseIndex].ReceiveIncoming(data);
            result = clause.AcceptVisitor(this, data);
            data = result.Variables;
            lastValidResult = result;
            if (result.KnownBoolResult == false) {
              backtrack = true;
            }
            if (result.ThrowsException) {
              //Don't backtrack. Exceptions completely stop the query.
              break;
            }
            else {
              afterClauseVariableStates [currentClauseIndex].ReceiveIncoming(data);
            }
          }

          if (backtrack) {
            int? newIndex;
            for (;;) {
              newIndex = backtracingClauses.LastOrDefault(index => index < currentClauseIndex);
              if (newIndex == null) {
                //We've reached the end
                break;
              }

              currentClauseIndex = (int)newIndex + 1;

              if (!beforeClauseVariableStates[currentClauseIndex].ReceiveIncoming(lastValidResult.Variables)) {
                newIndex = null;
                break;
              }
            }

            if (newIndex == null) {
              break;
            }

          } else {
            if (clause is QuerySelectClause) {
              outgoingData.ReceiveIncoming(data);
              if (outgoingEnumeratedValue == null)
                outgoingEnumeratedValue = result.EnumeratedValueResult;
              else
                outgoingEnumeratedValue = VariableStatusInfo.CombineStatus(outgoingEnumeratedValue.Value, result.EnumeratedValueResult);
            }

            ++currentClauseIndex;
          }
        }

        var finalData = new VariableStatusInfo();
        var endingClauseIndices = from item in clauses.Select((clause, i) => new { clause, i })
          let clause = item.clause
            where clause is QueryFromClause ||
          clause is QueryContinuationClause ||
          clause is QueryJoinClause ||
          clause is QuerySelectClause ||
          clause is QueryWhereClause
            select item.i;
        foreach (var clauseIndex in endingClauseIndices) {
          finalData.ReceiveIncoming(afterClauseVariableStates [clauseIndex]);
        }

        return VisitorResult.ForEnumeratedValue(finalData, outgoingEnumeratedValue ?? NullValueStatus.Unknown);
      }

      public override VisitorResult VisitQueryContinuationClause(QueryContinuationClause queryContinuationClause, VariableStatusInfo data)
      {
        return IntroduceVariableFromEnumeratedValue(queryContinuationClause.Identifier, queryContinuationClause.PrecedingQuery, data);
      }

      VisitorResult IntroduceVariableFromEnumeratedValue(string newVariable, Expression expression, VariableStatusInfo data)
      {
        var result = expression.AcceptVisitor(this, data);
        var newVariables = result.Variables.Clone();
        newVariables[newVariable] = result.EnumeratedValueResult;
        return VisitorResult.ForValue(newVariables, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitQueryFromClause(QueryFromClause queryFromClause, VariableStatusInfo data)
      {
        return IntroduceVariableFromEnumeratedValue(queryFromClause.Identifier, queryFromClause.Expression, data);
      }

      public override VisitorResult VisitQueryJoinClause(QueryJoinClause queryJoinClause, VariableStatusInfo data)
      {
        //TODO: Check if this really works in weird edge-cases.
        var tentativeResult = IntroduceVariableFromEnumeratedValue(queryJoinClause.JoinIdentifier, queryJoinClause.InExpression, data);
        tentativeResult = queryJoinClause.OnExpression.AcceptVisitor(this, tentativeResult.Variables);
        tentativeResult = queryJoinClause.EqualsExpression.AcceptVisitor(this, tentativeResult.Variables);

        if (queryJoinClause.IsGroupJoin) {
          var newVariables = tentativeResult.Variables.Clone();
          analysis.SetLocalVariableValue(newVariables, queryJoinClause.IntoIdentifierToken, NullValueStatus.DefinitelyNotNull);
          return VisitorResult.ForValue(newVariables, NullValueStatus.Unknown);
        }

        return tentativeResult;
      }

      public override VisitorResult VisitQueryLetClause(QueryLetClause queryLetClause, VariableStatusInfo data)
      {
        var result = queryLetClause.Expression.AcceptVisitor(this, data);

        string newVariable = queryLetClause.Identifier;
        var newVariables = result.Variables.Clone();
        newVariables [newVariable] = result.NullableReturnResult;

        return VisitorResult.ForValue(newVariables, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitQuerySelectClause(QuerySelectClause querySelectClause, VariableStatusInfo data)
      {
        var result = querySelectClause.Expression.AcceptVisitor(this, data);

        //The value of the expression in select becomes the "enumerated" value
        return VisitorResult.ForEnumeratedValue(result.Variables, result.NullableReturnResult);
      }

      public override VisitorResult VisitQueryWhereClause(QueryWhereClause queryWhereClause, VariableStatusInfo data)
      {
        var result = queryWhereClause.Condition.AcceptVisitor(this, data);

        return VisitorResult.ForEnumeratedValue(result.TruePathVariables, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitQueryOrderClause(QueryOrderClause queryOrderClause, VariableStatusInfo data)
      {
        foreach (var ordering in queryOrderClause.Orderings) {
          data = ordering.AcceptVisitor(this, data).Variables;
        }

        return VisitorResult.ForValue(data, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitQueryOrdering(QueryOrdering queryOrdering, VariableStatusInfo data)
      {
        return VisitorResult.ForValue(queryOrdering.Expression.AcceptVisitor(this, data).Variables, NullValueStatus.Unknown);
      }

      public override VisitorResult VisitQueryGroupClause(QueryGroupClause queryGroupClause, VariableStatusInfo data)
      {
        var projectionResult = queryGroupClause.Projection.AcceptVisitor(this, data);
        data = projectionResult.Variables;
        data = queryGroupClause.Key.AcceptVisitor(this, data).Variables;

        return VisitorResult.ForEnumeratedValue(data, projectionResult.NullableReturnResult);
      }
    }
  }
}