source: trunk/HeuristicLab.ExtLibs/HeuristicLab.NRefactory/5.5.0/NRefactory.CSharp-5.5.0/Parser/mcs/nullable.cs @ 16240

//
// nullable.cs: Nullable types support
//
// Authors: Martin Baulig (martin@ximian.com)
//          Miguel de Icaza (miguel@ximian.com)
//          Marek Safar (marek.safar@gmail.com)
//
// Dual licensed under the terms of the MIT X11 or GNU GPL
//
// Copyright 2001, 2002, 2003 Ximian, Inc (http://www.ximian.com)
// Copyright 2004-2008 Novell, Inc
// Copyright 2011 Xamarin Inc
//

using System;
using SLE = System.Linq.Expressions;

#if STATIC
using IKVM.Reflection.Emit;
#else
using System.Reflection.Emit;
#endif
  
namespace Mono.CSharp.Nullable
{
  public class NullableType : TypeExpr
  {
    readonly TypeSpec underlying;

    public NullableType (TypeSpec type, Location loc)
    {
      this.underlying = type;
      this.loc = loc;
    }

    public override TypeSpec ResolveAsType (IMemberContext ec, bool allowUnboundTypeArguments = false)
    {
      eclass = ExprClass.Type;

      var otype = ec.Module.PredefinedTypes.Nullable.Resolve ();
      if (otype == null)
        return null;

      TypeArguments args = new TypeArguments (new TypeExpression (underlying, loc));
      GenericTypeExpr ctype = new GenericTypeExpr (otype, args, loc);
      
      type = ctype.ResolveAsType (ec);
      return type;
    }
  }

  static class NullableInfo
  {
    public static MethodSpec GetConstructor (TypeSpec nullableType)
    {
      return (MethodSpec) MemberCache.FindMember (nullableType,
        MemberFilter.Constructor (ParametersCompiled.CreateFullyResolved (GetUnderlyingType (nullableType))), BindingRestriction.DeclaredOnly);
    }

    public static MethodSpec GetHasValue (TypeSpec nullableType)
    {
      return (MethodSpec) MemberCache.FindMember (nullableType,
        MemberFilter.Method ("get_HasValue", 0, ParametersCompiled.EmptyReadOnlyParameters, null), BindingRestriction.None);
    }

    public static MethodSpec GetGetValueOrDefault (TypeSpec nullableType)
    {
      return (MethodSpec) MemberCache.FindMember (nullableType,
        MemberFilter.Method ("GetValueOrDefault", 0, ParametersCompiled.EmptyReadOnlyParameters, null), BindingRestriction.None);
    }

    //
    // Don't use unless really required for correctness, see Unwrap::Emit
    //
    public static MethodSpec GetValue (TypeSpec nullableType)
    {
      return (MethodSpec) MemberCache.FindMember (nullableType,
        MemberFilter.Method ("get_Value", 0, ParametersCompiled.EmptyReadOnlyParameters, null), BindingRestriction.None);
    }

    public static TypeSpec GetUnderlyingType (TypeSpec nullableType)
    {
      return ((InflatedTypeSpec) nullableType).TypeArguments[0];
    }

    public static TypeSpec GetEnumUnderlyingType (ModuleContainer module, TypeSpec nullableEnum)
    {
      return MakeType (module, EnumSpec.GetUnderlyingType (GetUnderlyingType (nullableEnum)));
    }

    public static TypeSpec MakeType (ModuleContainer module, TypeSpec underlyingType)
    {
      return module.PredefinedTypes.Nullable.TypeSpec.MakeGenericType (module,
        new[] { underlyingType });

    }
  }

  public class Unwrap : Expression, IMemoryLocation
  {
    Expression expr;

    LocalTemporary temp;
    Expression temp_field;
    readonly bool useDefaultValue;

    public Unwrap (Expression expr, bool useDefaultValue = true)
    {
      this.expr = expr;
      this.loc = expr.Location;
      this.useDefaultValue = useDefaultValue;

      type = NullableInfo.GetUnderlyingType (expr.Type);
      eclass = expr.eclass;
    }

    public override bool ContainsEmitWithAwait ()
    {
      return expr.ContainsEmitWithAwait ();
    }

    // TODO: REMOVE
    public static Expression Create (Expression expr)
    {
      //
      // Avoid unwraping and wraping of same type
      //
      Wrap wrap = expr as Wrap;
      if (wrap != null)
        return wrap.Child;

      return Create (expr, false);
    }

    public static Expression CreateUnwrapped (Expression expr)
    {
      //
      // Avoid unwraping and wraping of same type
      //
      Wrap wrap = expr as Wrap;
      if (wrap != null)
        return wrap.Child;

      return Create (expr, true);
    }

    public static Unwrap Create (Expression expr, bool useDefaultValue)
    {
      return new Unwrap (expr, useDefaultValue);
    }
    
    public override Expression CreateExpressionTree (ResolveContext ec)
    {
      return expr.CreateExpressionTree (ec);
    }

    protected override Expression DoResolve (ResolveContext ec)
    {
      return this;
    }

    public override Expression DoResolveLValue (ResolveContext ec, Expression right_side)
    {
      expr = expr.DoResolveLValue (ec, right_side);
      return this;
    }

    public override void Emit (EmitContext ec)
    {
      Store (ec);

      var call = new CallEmitter ();
      call.InstanceExpression = this;

      //
      // Using GetGetValueOrDefault is prefered because JIT can possibly
      // inline it whereas Value property contains a throw which is very
      // unlikely to be inlined
      //
      if (useDefaultValue)
        call.EmitPredefined (ec, NullableInfo.GetGetValueOrDefault (expr.Type), null);
      else
        call.EmitPredefined (ec, NullableInfo.GetValue (expr.Type), null);
    }

    public void EmitCheck (EmitContext ec)
    {
      Store (ec);

      var call = new CallEmitter ();
      call.InstanceExpression = this;

      call.EmitPredefined (ec, NullableInfo.GetHasValue (expr.Type), null);
    }

    public override void EmitSideEffect (EmitContext ec)
    {
      expr.EmitSideEffect (ec);
    }

    public override Expression EmitToField (EmitContext ec)
    {
      if (temp_field == null)
        temp_field = this.expr.EmitToField (ec);
      
      return this;
    }

    public override bool Equals (object obj)
    {
      Unwrap uw = obj as Unwrap;
      return uw != null && expr.Equals (uw.expr);
    }

    public override void FlowAnalysis (FlowAnalysisContext fc)
    {
      expr.FlowAnalysis (fc);
    }

    public Expression Original {
      get {
        return expr;
      }
    }
    
    public override int GetHashCode ()
    {
      return expr.GetHashCode ();
    }

    public override bool IsNull {
      get {
        return expr.IsNull;
      }
    }

    public void Store (EmitContext ec)
    {
      if (temp != null || temp_field != null)
        return;

      if (expr is VariableReference)
        return;

      expr.Emit (ec);
      LocalVariable.Store (ec);
    }

    public void Load (EmitContext ec)
    {
      if (temp_field != null)
        temp_field.Emit (ec);
      else if (expr is VariableReference)
        expr.Emit (ec);
      else
        LocalVariable.Emit (ec);
    }

    public override SLE.Expression MakeExpression (BuilderContext ctx)
    {
      return expr.MakeExpression (ctx);
    }

    public void AddressOf (EmitContext ec, AddressOp mode)
    {
      IMemoryLocation ml;

      if (temp_field != null) {
        ml = temp_field as IMemoryLocation;
        if (ml == null) {
          var lt = new LocalTemporary (temp_field.Type);
          temp_field.Emit (ec);
          lt.Store (ec);
          ml = lt;
        }
      } else {
        ml = expr as VariableReference;
      }

      if (ml != null)
        ml.AddressOf (ec, mode);
      else
        LocalVariable.AddressOf (ec, mode);
    }

    //
    // Keeps result of non-variable expression
    //
    LocalTemporary LocalVariable {
      get {
        if (temp == null && temp_field == null)
          temp = new LocalTemporary (expr.Type);
        return temp;
      }
    }
  }

  //
  // Calls get_Value method on nullable expression
  //
  public class UnwrapCall : CompositeExpression
  {
    public UnwrapCall (Expression expr)
      : base (expr)
    {
    }

    protected override Expression DoResolve (ResolveContext rc)
    {
      base.DoResolve (rc);

      if (type != null)
        type = NullableInfo.GetUnderlyingType (type);

      return this;
    }

    public override void Emit (EmitContext ec)
    {
      var call = new CallEmitter ();
      call.InstanceExpression = Child;
      call.EmitPredefined (ec, NullableInfo.GetValue (Child.Type), null);
    }
  }

  public class Wrap : TypeCast
  {
    private Wrap (Expression expr, TypeSpec type)
      : base (expr, type)
    {
      eclass = ExprClass.Value;
    }

    public override Expression CreateExpressionTree (ResolveContext ec)
    {
      TypeCast child_cast = child as TypeCast;
      if (child_cast != null) {
        child.Type = type;
        return child_cast.CreateExpressionTree (ec);
      }

      var user_cast = child as UserCast;
      if (user_cast != null) {
        child.Type = type;
        return user_cast.CreateExpressionTree (ec);
      }

      return base.CreateExpressionTree (ec);
    }

    public static Expression Create (Expression expr, TypeSpec type)
    {
      //
      // Avoid unwraping and wraping of the same type
      //
      Unwrap unwrap = expr as Unwrap;
      if (unwrap != null && expr.Type == NullableInfo.GetUnderlyingType (type))
        return unwrap.Original;
    
      return new Wrap (expr, type);
    }
    
    public override void Emit (EmitContext ec)
    {
      child.Emit (ec);
      ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));
    }
  }

  //
  // Represents null literal lifted to nullable type
  //
  public class LiftedNull : NullConstant, IMemoryLocation
  {
    private LiftedNull (TypeSpec nullable_type, Location loc)
      : base (nullable_type, loc)
    {
      eclass = ExprClass.Value;
    }

    public static Constant Create (TypeSpec nullable, Location loc)
    {
      return new LiftedNull (nullable, loc);
    }

    public static Constant CreateFromExpression (ResolveContext rc, Expression e)
    {
      if (!rc.HasSet (ResolveContext.Options.ExpressionTreeConversion)) {
        rc.Report.Warning (458, 2, e.Location, "The result of the expression is always `null' of type `{0}'",
          e.Type.GetSignatureForError ());
      }

      return ReducedExpression.Create (Create (e.Type, e.Location), e);
    }

    public override void Emit (EmitContext ec)
    {
      // TODO: generate less temporary variables
      LocalTemporary value_target = new LocalTemporary (type);

      value_target.AddressOf (ec, AddressOp.Store);
      ec.Emit (OpCodes.Initobj, type);
      value_target.Emit (ec);
      value_target.Release (ec);
    }

    public void AddressOf (EmitContext ec, AddressOp Mode)
    {
      LocalTemporary value_target = new LocalTemporary (type);
        
      value_target.AddressOf (ec, AddressOp.Store);
      ec.Emit (OpCodes.Initobj, type);
      value_target.AddressOf (ec, Mode);
    }
  }

  //
  // Generic lifting expression, supports all S/S? -> T/T? cases
  //
  public class LiftedConversion : Expression, IMemoryLocation
  {
    Expression expr, null_value;
    Unwrap unwrap;

    public LiftedConversion (Expression expr, Unwrap unwrap, TypeSpec type)
    {
      this.expr = expr;
      this.unwrap = unwrap;
      this.loc = expr.Location;
      this.type = type;
    }

    public LiftedConversion (Expression expr, Expression unwrap, TypeSpec type)
      : this (expr, unwrap as Unwrap, type)
    {
    }

    public override bool IsNull {
      get {
        return expr.IsNull;
      }
    }

    public override bool ContainsEmitWithAwait ()
    {
      return unwrap.ContainsEmitWithAwait ();
    }
    
    public override Expression CreateExpressionTree (ResolveContext ec)
    {
      return expr.CreateExpressionTree (ec);
    }     

    protected override Expression DoResolve (ResolveContext ec)
    {
      //
      // It's null when lifting non-nullable type
      //
      if (unwrap == null) {
        // S -> T? is wrap only
        if (type.IsNullableType)
          return Wrap.Create (expr, type);

        // S -> T can be simplified
        return expr;
      }

      // Wrap target for T?
      if (type.IsNullableType) {
        if (!expr.Type.IsNullableType) {
          expr = Wrap.Create (expr, type);
          if (expr == null)
            return null;
        }

        null_value = LiftedNull.Create (type, loc);
      } else if (TypeSpec.IsValueType (type)) {
        null_value = LiftedNull.Create (type, loc);
      } else {
        null_value = new NullConstant (type, loc);
      }

      eclass = ExprClass.Value;
      return this;
    }

    public override void Emit (EmitContext ec)
    {
      Label is_null_label = ec.DefineLabel ();
      Label end_label = ec.DefineLabel ();

      unwrap.EmitCheck (ec);
      ec.Emit (OpCodes.Brfalse, is_null_label);

      expr.Emit (ec);

      ec.Emit (OpCodes.Br, end_label);
      ec.MarkLabel (is_null_label);

      null_value.Emit (ec);

      ec.MarkLabel (end_label);
    }

    public override void FlowAnalysis (FlowAnalysisContext fc)
    {
      expr.FlowAnalysis (fc);
    }

    public void AddressOf (EmitContext ec, AddressOp mode)
    {
      unwrap.AddressOf (ec, mode);
    }
  }

  public class LiftedUnaryOperator : Unary, IMemoryLocation
  {
    Unwrap unwrap;
    Expression user_operator;

    public LiftedUnaryOperator (Unary.Operator op, Expression expr, Location loc)
      : base (op, expr, loc)
    {
    }

    public void AddressOf (EmitContext ec, AddressOp mode)
    {
      unwrap.AddressOf (ec, mode);
    }

    public override Expression CreateExpressionTree (ResolveContext ec)
    {
      if (user_operator != null)
        return user_operator.CreateExpressionTree (ec);

      if (Oper == Operator.UnaryPlus)
        return Expr.CreateExpressionTree (ec);

      return base.CreateExpressionTree (ec);
    }

    protected override Expression DoResolve (ResolveContext ec)
    {
      unwrap = Unwrap.Create (Expr, false);
      if (unwrap == null)
        return null;

      Expression res = base.ResolveOperator (ec, unwrap);
      if (res == null) {
        Error_OperatorCannotBeApplied (ec, loc, OperName (Oper), Expr.Type);
        return null;
      }

      if (res != this) {
        if (user_operator == null)
          return res;
      } else {
        res = Expr = LiftExpression (ec, Expr);
      }

      if (res == null)
        return null;

      eclass = ExprClass.Value;
      type = res.Type;
      return this;
    }

    public override void Emit (EmitContext ec)
    {
      Label is_null_label = ec.DefineLabel ();
      Label end_label = ec.DefineLabel ();

      unwrap.EmitCheck (ec);
      ec.Emit (OpCodes.Brfalse, is_null_label);

      if (user_operator != null) {
        user_operator.Emit (ec);
      } else {
        EmitOperator (ec, NullableInfo.GetUnderlyingType (type));
      }

      ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));
      ec.Emit (OpCodes.Br_S, end_label);

      ec.MarkLabel (is_null_label);
      LiftedNull.Create (type, loc).Emit (ec);

      ec.MarkLabel (end_label);
    }

    static Expression LiftExpression (ResolveContext ec, Expression expr)
    {
      var lifted_type = new NullableType (expr.Type, expr.Location);
      if (lifted_type.ResolveAsType (ec) == null)
        return null;

      expr.Type = lifted_type.Type;
      return expr;
    }

    protected override Expression ResolveEnumOperator (ResolveContext ec, Expression expr, TypeSpec[] predefined)
    {
      expr = base.ResolveEnumOperator (ec, expr, predefined);
      if (expr == null)
        return null;

      Expr = LiftExpression (ec, Expr);
      return LiftExpression (ec, expr);
    }

    protected override Expression ResolveUserOperator (ResolveContext ec, Expression expr)
    {
      expr = base.ResolveUserOperator (ec, expr);
      if (expr == null)
        return null;

      //
      // When a user operator is of non-nullable type
      //
      if (Expr is Unwrap) {
        user_operator = LiftExpression (ec, expr);
        return user_operator;
      }

      return expr;
    }
  }

  //
  // Lifted version of binary operators
  //
  class LiftedBinaryOperator : Expression
  {
    public LiftedBinaryOperator (Binary b)
    {
      this.Binary = b;
      this.loc = b.Location;
    }

    public Binary Binary { get; private set; }

    public Expression Left { get; set; }

    public Expression Right { get; set; }

    public Unwrap UnwrapLeft { get; set; }

    public Unwrap UnwrapRight { get; set; }

    public MethodSpec UserOperator { get; set; }

    bool IsBitwiseBoolean {
      get {
        return (Binary.Oper == Binary.Operator.BitwiseAnd || Binary.Oper == Binary.Operator.BitwiseOr) &&
        ((UnwrapLeft != null && UnwrapLeft.Type.BuiltinType == BuiltinTypeSpec.Type.Bool) ||
         (UnwrapRight != null && UnwrapRight.Type.BuiltinType == BuiltinTypeSpec.Type.Bool));
      }
    }

    public override bool ContainsEmitWithAwait ()
    {
      return Left.ContainsEmitWithAwait () || Right.ContainsEmitWithAwait ();
    }

    public override Expression CreateExpressionTree (ResolveContext rc)
    {
      if (UserOperator != null) {
        Arguments args = new Arguments (2);
        args.Add (new Argument (Binary.Left));
        args.Add (new Argument (Binary.Right));

        var method = new UserOperatorCall (UserOperator, args, Binary.CreateExpressionTree, loc);
        return method.CreateExpressionTree (rc);
      }

      return Binary.CreateExpressionTree (rc);
    }

    protected override Expression DoResolve (ResolveContext rc)
    {
      if (rc.IsRuntimeBinder) {
        if (UnwrapLeft == null && !Left.Type.IsNullableType)
          Left = LiftOperand (rc, Left);

        if (UnwrapRight == null && !Right.Type.IsNullableType)
          Right = LiftOperand (rc, Right);
      } else {
        if (UnwrapLeft == null && Left != null && Left.Type.IsNullableType) {
          Left = Unwrap.CreateUnwrapped (Left);
          UnwrapLeft = Left as Unwrap;
        }

        if (UnwrapRight == null && Right != null && Right.Type.IsNullableType) {
          Right = Unwrap.CreateUnwrapped (Right);
          UnwrapRight = Right as Unwrap;
        }
      }

      type = Binary.Type;
      eclass = Binary.eclass; 

      return this;
    }

    Expression LiftOperand (ResolveContext rc, Expression expr)
    {
      TypeSpec type;
      if (expr.IsNull) {
        type = Left.IsNull ? Right.Type : Left.Type;
      } else {
        type = expr.Type;
      }

      if (!type.IsNullableType)
        type = NullableInfo.MakeType (rc.Module, type);

      return Wrap.Create (expr, type);
    }

    public override void Emit (EmitContext ec)
    {
      if (IsBitwiseBoolean && UserOperator == null) {
        EmitBitwiseBoolean (ec);
        return;
      }

      if ((Binary.Oper & Binary.Operator.EqualityMask) != 0) {
        EmitEquality (ec);
        return;
      }

      Label is_null_label = ec.DefineLabel ();
      Label end_label = ec.DefineLabel ();

      if (ec.HasSet (BuilderContext.Options.AsyncBody) && Right.ContainsEmitWithAwait ()) {
        Left = Left.EmitToField (ec);
        Right = Right.EmitToField (ec);
      }

      if (UnwrapLeft != null) {
        UnwrapLeft.EmitCheck (ec);
      }

      //
      // Don't emit HasValue check when left and right expressions are same
      //
      if (UnwrapRight != null && !Binary.Left.Equals (Binary.Right)) {
        UnwrapRight.EmitCheck (ec);
        if (UnwrapLeft != null) {
          ec.Emit (OpCodes.And);
        }
      }

      ec.Emit (OpCodes.Brfalse, is_null_label);

      if (UserOperator != null) {
        var args = new Arguments (2);
        args.Add (new Argument (Left));
        args.Add (new Argument (Right));

        var call = new CallEmitter ();
        call.EmitPredefined (ec, UserOperator, args);
      } else {
        Binary.EmitOperator (ec, Left, Right);
      }

      //
      // Wrap the result when the operator return type is nullable type
      //
      if (type.IsNullableType)
        ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));

      ec.Emit (OpCodes.Br_S, end_label);
      ec.MarkLabel (is_null_label);

      if ((Binary.Oper & Binary.Operator.ComparisonMask) != 0) {
        ec.EmitInt (0);
      } else {
        LiftedNull.Create (type, loc).Emit (ec);
      }

      ec.MarkLabel (end_label);
    }

    void EmitBitwiseBoolean (EmitContext ec)
    {
      Label load_left = ec.DefineLabel ();
      Label load_right = ec.DefineLabel ();
      Label end_label = ec.DefineLabel ();
      Label is_null_label = ec.DefineLabel ();

      bool or = Binary.Oper == Binary.Operator.BitwiseOr;

      //
      // Both operands are bool? types
      //
      if (UnwrapLeft != null && UnwrapRight != null) {
        if (ec.HasSet (BuilderContext.Options.AsyncBody) && Binary.Right.ContainsEmitWithAwait ()) {
          Left = Left.EmitToField (ec);
          Right = Right.EmitToField (ec);
        } else {
          UnwrapLeft.Store (ec);
          UnwrapRight.Store (ec);
        }

        Left.Emit (ec);
        ec.Emit (OpCodes.Brtrue_S, load_right);

        Right.Emit (ec);
        ec.Emit (OpCodes.Brtrue_S, load_left);

        UnwrapLeft.EmitCheck (ec);
        ec.Emit (OpCodes.Brfalse_S, load_right);

        // load left
        ec.MarkLabel (load_left);
        if (or)
          UnwrapRight.Load (ec);
        else
          UnwrapLeft.Load (ec);

        ec.Emit (OpCodes.Br_S, end_label);

        // load right
        ec.MarkLabel (load_right);
        if (or)
          UnwrapLeft.Load (ec);
        else
          UnwrapRight.Load (ec);

        ec.MarkLabel (end_label);
        return;
      }

      //
      // Faster version when one operand is bool
      //
      if (UnwrapLeft == null) {
        //
        // (bool, bool?)
        //
        // Optimizes remaining (false & bool?), (true | bool?) which are not easy to handle
        // in binary expression reduction
        //
        var c = Left as BoolConstant;
        if (c != null) {
          // Keep evaluation order
          UnwrapRight.Store (ec);

          ec.EmitInt (or ? 1 : 0);
          ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));
        } else if (Left.IsNull) {
          UnwrapRight.Emit (ec);
          ec.Emit (or ? OpCodes.Brfalse_S : OpCodes.Brtrue_S, is_null_label);

          UnwrapRight.Load (ec);
          ec.Emit (OpCodes.Br_S, end_label);

          ec.MarkLabel (is_null_label);
          LiftedNull.Create (type, loc).Emit (ec);
        } else {
          Left.Emit (ec);
          ec.Emit (or ? OpCodes.Brfalse_S : OpCodes.Brtrue_S, load_right);

          ec.EmitInt (or ? 1 : 0);
          ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));

          ec.Emit (OpCodes.Br_S, end_label);

          ec.MarkLabel (load_right);
          UnwrapRight.Original.Emit (ec);
        }
      } else {
        //
        // (bool?, bool)
        //
        // Keep left-right evaluation order
        UnwrapLeft.Store (ec);

        //
        // Optimizes remaining (bool? & false), (bool? | true) which are not easy to handle
        // in binary expression reduction
        //
        var c = Right as BoolConstant;
        if (c != null) {
          ec.EmitInt (or ? 1 : 0);
          ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));
        } else if (Right.IsNull) {
          UnwrapLeft.Emit (ec);
          ec.Emit (or ? OpCodes.Brfalse_S : OpCodes.Brtrue_S, is_null_label);

          UnwrapLeft.Load (ec);
          ec.Emit (OpCodes.Br_S, end_label);

          ec.MarkLabel (is_null_label);
          LiftedNull.Create (type, loc).Emit (ec);
        } else {
          Right.Emit (ec);
          ec.Emit (or ? OpCodes.Brfalse_S : OpCodes.Brtrue_S, load_right);

          ec.EmitInt (or ? 1 : 0);
          ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));

          ec.Emit (OpCodes.Br_S, end_label);

          ec.MarkLabel (load_right);

          UnwrapLeft.Load (ec);
        }
      }

      ec.MarkLabel (end_label);
    }

    //
    // Emits optimized equality or inequality operator when possible
    //
    void EmitEquality (EmitContext ec)
    {
      //
      // Either left or right is null
      // 
      if (UnwrapLeft != null && Binary.Right.IsNull) { // TODO: Optimize for EmitBranchable
        //
        // left.HasValue == false 
        //
        UnwrapLeft.EmitCheck (ec);
        if (Binary.Oper == Binary.Operator.Equality) {
          ec.EmitInt (0);
          ec.Emit (OpCodes.Ceq);
        }
        return;
      }

      if (UnwrapRight != null && Binary.Left.IsNull) {
        //
        // right.HasValue == false 
        //
        UnwrapRight.EmitCheck (ec);
        if (Binary.Oper == Binary.Operator.Equality) {
          ec.EmitInt (0);
          ec.Emit (OpCodes.Ceq);
        }
        return;
      }

      Label dissimilar_label = ec.DefineLabel ();
      Label end_label = ec.DefineLabel ();

      if (UserOperator != null) {
        var left = Left;

        if (UnwrapLeft != null) {
          UnwrapLeft.EmitCheck (ec);
        } else {
          // Keep evaluation order same
          if (!(Left is VariableReference)) {
            Left.Emit (ec);
            var lt = new LocalTemporary (Left.Type);
            lt.Store (ec);
            left = lt;
          }
        }

        if (UnwrapRight != null) {
          UnwrapRight.EmitCheck (ec);

          if (UnwrapLeft != null) {
            ec.Emit (OpCodes.Bne_Un, dissimilar_label);

            Label compare_label = ec.DefineLabel ();
            UnwrapLeft.EmitCheck (ec);
            ec.Emit (OpCodes.Brtrue, compare_label);

            if (Binary.Oper == Binary.Operator.Equality)
              ec.EmitInt (1);
            else
              ec.EmitInt (0);

            ec.Emit (OpCodes.Br, end_label);

            ec.MarkLabel (compare_label);
          } else {
            ec.Emit (OpCodes.Brfalse, dissimilar_label);
          }
        } else {
          ec.Emit (OpCodes.Brfalse, dissimilar_label);
        }

        var args = new Arguments (2);
        args.Add (new Argument (left));
        args.Add (new Argument (Right));

        var call = new CallEmitter ();
        call.EmitPredefined (ec, UserOperator, args);
      } else {
        if (ec.HasSet (BuilderContext.Options.AsyncBody) && Binary.Right.ContainsEmitWithAwait ()) {
          Left = Left.EmitToField (ec);
          Right = Right.EmitToField (ec);
        }

        //
        // Emit underlying value comparison first.
        //
        // For this code: int? a = 1; bool b = a == 1;
        //
        // We emit something similar to this. Expressions with side effects have local
        // variable created by Unwrap expression
        //
        //  left.GetValueOrDefault ()
        //  right
        //  bne.un.s   dissimilar_label
        //  left.HasValue
        //  br.s       end_label
        // dissimilar_label:
        //  ldc.i4.0
        // end_label:
        //

        Left.Emit (ec);
        Right.Emit (ec);

        ec.Emit (OpCodes.Bne_Un_S, dissimilar_label);

        //
        // Check both left and right expressions for Unwrap call in which
        // case we need to run get_HasValue() check because the type is
        // nullable and could have null value
        //
        if (UnwrapLeft != null)
          UnwrapLeft.EmitCheck (ec);

        if (UnwrapRight != null)
          UnwrapRight.EmitCheck (ec);

        if (UnwrapLeft != null && UnwrapRight != null) {
          if (Binary.Oper == Binary.Operator.Inequality)
            ec.Emit (OpCodes.Xor);
          else
            ec.Emit (OpCodes.Ceq);
        } else {
          if (Binary.Oper == Binary.Operator.Inequality) {
            ec.EmitInt (0);
            ec.Emit (OpCodes.Ceq);
          }
        }
      }

      ec.Emit (OpCodes.Br_S, end_label);

      ec.MarkLabel (dissimilar_label);
      if (Binary.Oper == Binary.Operator.Inequality)
        ec.EmitInt (1);
      else
        ec.EmitInt (0);

      ec.MarkLabel (end_label);
    }

    public override void FlowAnalysis (FlowAnalysisContext fc)
    {
      Binary.FlowAnalysis (fc);
    }

    public override SLE.Expression MakeExpression (BuilderContext ctx)
    {
      return Binary.MakeExpression (ctx, Left, Right);
    }
  }

  public class NullCoalescingOperator : Expression
  {
    Expression left, right;
    Unwrap unwrap;

    public NullCoalescingOperator (Expression left, Expression right)
    {
      this.left = left;
      this.right = right;
      this.loc = left.Location;
    }

    public Expression LeftExpression {
      get {
        return left;
      }
    }

    public Expression RightExpression {
      get {
        return right;
      }
    }
    
    public override Expression CreateExpressionTree (ResolveContext ec)
    {
      if (left is NullLiteral)
        ec.Report.Error (845, loc, "An expression tree cannot contain a coalescing operator with null left side");

      UserCast uc = left as UserCast;
      Expression conversion = null;
      if (uc != null) {
        left = uc.Source;

        Arguments c_args = new Arguments (2);
        c_args.Add (new Argument (uc.CreateExpressionTree (ec)));
        c_args.Add (new Argument (left.CreateExpressionTree (ec)));
        conversion = CreateExpressionFactoryCall (ec, "Lambda", c_args);
      }

      Arguments args = new Arguments (3);
      args.Add (new Argument (left.CreateExpressionTree (ec)));
      args.Add (new Argument (right.CreateExpressionTree (ec)));
      if (conversion != null)
        args.Add (new Argument (conversion));
      
      return CreateExpressionFactoryCall (ec, "Coalesce", args);
    }

    Expression ConvertExpression (ResolveContext ec)
    {
      // TODO: ImplicitConversionExists should take care of this
      if (left.eclass == ExprClass.MethodGroup)
        return null;

      TypeSpec ltype = left.Type;

      //
      // If left is a nullable type and an implicit conversion exists from right to underlying type of left,
      // the result is underlying type of left
      //
      if (ltype.IsNullableType) {
        unwrap = Unwrap.Create (left, false);
        if (unwrap == null)
          return null;

        //
        // Reduce (left ?? null) to left
        //
        if (right.IsNull)
          return ReducedExpression.Create (left, this);

        Expression conv;
        if (right.Type.IsNullableType) {
          conv = right.Type == ltype ? right : Convert.ImplicitNulableConversion (ec, right, ltype);
          if (conv != null) {
            right = conv;
            type = ltype;
            return this;
          }
        } else {
          conv = Convert.ImplicitConversion (ec, right, unwrap.Type, loc);
          if (conv != null) {
            left = unwrap;
            ltype = left.Type;

            //
            // If right is a dynamic expression, the result type is dynamic
            //
            if (right.Type.BuiltinType == BuiltinTypeSpec.Type.Dynamic) {
              type = right.Type;

              // Need to box underlying value type
              left = Convert.ImplicitBoxingConversion (left, ltype, type);
              return this;
            }

            right = conv;
            type = ltype;
            return this;
          }
        }
      } else if (TypeSpec.IsReferenceType (ltype)) {
        if (Convert.ImplicitConversionExists (ec, right, ltype)) {
          //
          // If right is a dynamic expression, the result type is dynamic
          //
          if (right.Type.BuiltinType == BuiltinTypeSpec.Type.Dynamic) {
            type = right.Type;
            return this;
          }

          //
          // Reduce ("foo" ?? expr) to expression
          //
          Constant lc = left as Constant;
          if (lc != null && !lc.IsDefaultValue)
            return ReducedExpression.Create (lc, this, false);

          //
          // Reduce (left ?? null) to left OR (null-constant ?? right) to right
          //
          if (right.IsNull || lc != null) {
            //
            // Special case null ?? null
            //
            if (right.IsNull && ltype == right.Type)
              return null;

            return ReducedExpression.Create (lc != null ? right : left, this, false);
          }

          right = Convert.ImplicitConversion (ec, right, ltype, loc);
          type = ltype;
          return this;
        }
      } else {
        return null;
      }

      TypeSpec rtype = right.Type;
      if (!Convert.ImplicitConversionExists (ec, unwrap ?? left, rtype) || right.eclass == ExprClass.MethodGroup)
        return null;

      //
      // Reduce (null ?? right) to right
      //
      if (left.IsNull)
        return ReducedExpression.Create (right, this, false).Resolve (ec);

      left = Convert.ImplicitConversion (ec, unwrap ?? left, rtype, loc);

      if (TypeSpec.IsValueType (left.Type) && !left.Type.IsNullableType) {
        Warning_UnreachableExpression (ec, right.Location);
        return ReducedExpression.Create (left, this, false).Resolve (ec);
      }

      type = rtype;
      return this;
    }

    public override bool ContainsEmitWithAwait ()
    {
      if (unwrap != null)
        return unwrap.ContainsEmitWithAwait () || right.ContainsEmitWithAwait ();

      return left.ContainsEmitWithAwait () || right.ContainsEmitWithAwait ();
    }

    protected override Expression DoResolve (ResolveContext ec)
    {
      left = left.Resolve (ec);
      right = right.Resolve (ec);

      if (left == null || right == null)
        return null;

      eclass = ExprClass.Value;

      Expression e = ConvertExpression (ec);
      if (e == null) {
        Binary.Error_OperatorCannotBeApplied (ec, left, right, "??", loc);
        return null;
      }

      return e;
    }

    public override void Emit (EmitContext ec)
    {
      Label end_label = ec.DefineLabel ();

      if (unwrap != null) {
        Label is_null_label = ec.DefineLabel ();

        unwrap.EmitCheck (ec);
        ec.Emit (OpCodes.Brfalse, is_null_label);

        //
        // When both expressions are nullable the unwrap
        // is needed only for null check not for value uwrap
        //
        if (type.IsNullableType && TypeSpecComparer.IsEqual (NullableInfo.GetUnderlyingType (type), unwrap.Type))
          unwrap.Load (ec);
        else
          left.Emit (ec);

        ec.Emit (OpCodes.Br, end_label);

        ec.MarkLabel (is_null_label);
        right.Emit (ec);

        ec.MarkLabel (end_label);
        return;
      }

      left.Emit (ec);
      ec.Emit (OpCodes.Dup);

      // Only to make verifier happy
      if (left.Type.IsGenericParameter)
        ec.Emit (OpCodes.Box, left.Type);

      ec.Emit (OpCodes.Brtrue, end_label);

      ec.Emit (OpCodes.Pop);
      right.Emit (ec);

      ec.MarkLabel (end_label);
    }

    public override void FlowAnalysis (FlowAnalysisContext fc)
    {
      left.FlowAnalysis (fc);
      var left_da = fc.BranchDefiniteAssignment ();
      right.FlowAnalysis (fc);
      fc.DefiniteAssignment = left_da;
    }

    protected override void CloneTo (CloneContext clonectx, Expression t)
    {
      NullCoalescingOperator target = (NullCoalescingOperator) t;

      target.left = left.Clone (clonectx);
      target.right = right.Clone (clonectx);
    }
    
    public override object Accept (StructuralVisitor visitor)
    {
      return visitor.Visit (this);
    }
  }

  class LiftedUnaryMutator : UnaryMutator
  {
    public LiftedUnaryMutator (Mode mode, Expression expr, Location loc)
      : base (mode, expr, loc)
    {
    }

    protected override Expression DoResolve (ResolveContext ec)
    {
      var orig_expr = expr;

      expr = Unwrap.Create (expr);

      var res = base.DoResolveOperation (ec);

      expr = orig_expr;
      type = expr.Type;

      return res;
    }

    protected override void EmitOperation (EmitContext ec)
    {
      Label is_null_label = ec.DefineLabel ();
      Label end_label = ec.DefineLabel ();

      LocalTemporary lt = new LocalTemporary (type);

      // Value is on the stack
      lt.Store (ec);

      var call = new CallEmitter ();
      call.InstanceExpression = lt;
      call.EmitPredefined (ec, NullableInfo.GetHasValue (expr.Type), null);

      ec.Emit (OpCodes.Brfalse, is_null_label);

      call = new CallEmitter ();
      call.InstanceExpression = lt;
      call.EmitPredefined (ec, NullableInfo.GetGetValueOrDefault (expr.Type), null);

      lt.Release (ec);

      base.EmitOperation (ec);

      ec.Emit (OpCodes.Newobj, NullableInfo.GetConstructor (type));
      ec.Emit (OpCodes.Br_S, end_label);

      ec.MarkLabel (is_null_label);
      LiftedNull.Create (type, loc).Emit (ec);

      ec.MarkLabel (end_label);
    }
  }
}