private BoundExpression DistributeLiftedConversionIntoLiftedOperand(
            CSharpSyntaxNode syntax,
            BoundExpression operand,
            ConversionKind kind,
            bool @checked,
            MethodSymbol method,
            TypeSymbol type)
        {
            // Third, an even trickier optimization. Suppose we have a lifted conversion on top of
            // a lifted operation. Say, "decimal? d = M() + N()" where M() and N() return nullable ints.
            // We can codegen this naively as:
            //
            // int? m = M();
            // int? n = N();
            // int? r = m.HasValue && n.HasValue ? new int?(m.Value + n.Value) : new int?();
            // decimal? d = r.HasValue ? new decimal?((decimal)r.Value) : new decimal?();
            //
            // However, we also observe that we could do the conversion on both branches of the conditional:
            //
            // int? m = M();
            // int? n = N();
            // decimal? d = m.HasValue && n.HasValue ? (decimal?)(new int?(m.Value + n.Value)) : (decimal?)(new int?());
            //
            // And we already optimize those, above! So we could reduce this to:
            //
            // int? m = M();
            // int? n = N();
            // decimal? d = m.HasValue && n.HasValue ? new decimal?((decimal)(m.Value + n.Value)) : new decimal?());
            //
            // which avoids entirely the creation of the unnecessary nullable int!

            if (operand.Kind == BoundKind.Sequence)
            {
                BoundSequence seq = (BoundSequence)operand;
                if (seq.Value.Kind == BoundKind.ConditionalOperator)
                {
                    BoundConditionalOperator conditional = (BoundConditionalOperator)seq.Value;
                    Debug.Assert(seq.Type == conditional.Type);
                    Debug.Assert(conditional.Type == conditional.Consequence.Type);
                    Debug.Assert(conditional.Type == conditional.Alternative.Type);

                    if (NullableAlwaysHasValue(conditional.Consequence) != null && NullableNeverHasValue(conditional.Alternative))
                    {
                        return new BoundSequence(
                            seq.Syntax,
                            seq.Locals,
                            seq.SideEffects,
                            RewriteConditionalOperator(
                                conditional.Syntax,
                                conditional.Condition,
                                MakeConversion(null, syntax, conditional.Consequence, kind, method, @checked, false, false, false, ConstantValue.NotAvailable, type),
                                MakeConversion(null, syntax, conditional.Alternative, kind, method, @checked, false, false, false, ConstantValue.NotAvailable, type),
                                ConstantValue.NotAvailable,
                                type),
                            type);
                    }
                }
            }

            return null;
        }

        internal static ConstantValue GetAsOperatorConstantResult(TypeSymbol operandType, TypeSymbol targetType, ConversionKind conversionKind, ConstantValue operandConstantValue)
        {
            // NOTE:    Even though BoundIsOperator and BoundAsOperator will always have no ConstantValue
            // NOTE:    (they are non-constant expressions according to Section 7.19 of the specification),
            // NOTE:    we want to perform constant analysis of is/as expressions during binding to generate warnings (always true/false/null)
            // NOTE:    and during rewriting for optimized codegen.

            // Native compiler port:
            //     // check for case we know is always false
            //     if (arg->isNull() || !canCast(arg, type2, NOUDC) && type1->IsValType() && type2->isClassType() && (!type1->IsTypeParameterType() || !type2->isPredefType(PT_ENUM)))
            //     {
            //          GetErrorContext()->Error(tree, WRN_AlwaysNull, type2);
            //          return rval;
            //     }

            if (operandConstantValue == ConstantValue.Null ||
                (conversionKind == ConversionKind.NoConversion &&
                 (operandType.IsValueType && targetType.IsClassType() && (!operandType.IsTypeParameter() || targetType.SpecialType != SpecialType.System_Enum))))
            {
                return ConstantValue.Null;
            }
            else
            {
                return null;
            }
        }

        private BoundExpression OptimizeLiftedBuiltInConversion(
            CSharpSyntaxNode syntax,
            BoundExpression operand,
            ConversionKind kind,
            bool @checked,
            TypeSymbol type)
        {
            Debug.Assert(operand != null);
            Debug.Assert((object)type != null);

            // First, an optimization. If the source is known to always be null then
            // we can simply return the alternative.

            if (NullableNeverHasValue(operand))
            {
                return new BoundDefaultOperator(syntax, null, type);
            }

            // Second, a trickier optimization. If the conversion is "(T?)(new S?(x))" then
            // we generate "new T?((T)x)"

            BoundExpression nonNullValue = NullableAlwaysHasValue(operand);
            if (nonNullValue != null)
            {
                return new BoundObjectCreationExpression(
                    syntax,
                    GetNullableMethod(syntax, type, SpecialMember.System_Nullable_T__ctor),
                    MakeConversion(
                        nonNullValue,
                        type.GetNullableUnderlyingType(),
                        @checked));
            }

            // Third, a very tricky optimization.
            return DistributeLiftedConversionIntoLiftedOperand(syntax, operand, kind, @checked, null, type);
        }

        /// <summary>
        /// Helper method to generate a lowered conversion.
        /// </summary>
        private BoundExpression MakeConversion(
            BoundConversion oldNode,
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            ConversionKind conversionKind,
            MethodSymbol symbolOpt,
            bool @checked,
            bool explicitCastInCode,
            bool isExtensionMethod,
            bool isArrayIndex,
            ConstantValue constantValueOpt,
            TypeSymbol rewrittenType)
        {
            Debug.Assert(oldNode == null || oldNode.Syntax == syntax);
            Debug.Assert((object)rewrittenType != null);
            @checked = @checked &&
                (inExpressionLambda && (explicitCastInCode || DistinctSpecialTypes(rewrittenOperand.Type, rewrittenType)) ||
                NeedsChecked(rewrittenOperand.Type, rewrittenType));

            switch (conversionKind)
            {
                case ConversionKind.Identity:

                    // Spec 6.1.1:
                    //   An identity conversion converts from any type to the same type. 
                    //   This conversion exists such that an entity that already has a required type can be said to be convertible to that type.
                    //   Because object and dynamic are considered equivalent there is an identity conversion between object and dynamic, 
                    //   and between constructed types that are the same when replacing all occurrences of dynamic with object.

                    // Why ignoreDynamic: false?
                    // Lowering phase treats object and dynamic as equivalent types. So we don't need to produce any conversion here,
                    // but we need to change the Type property on the resulting BoundExpression to match the rewrittenType.
                    // This is necessary so that subsequent lowering transformations see that the expression is dynamic.

                    if (inExpressionLambda || !rewrittenOperand.Type.Equals(rewrittenType, ignoreCustomModifiers: false, ignoreDynamic: false))
                    {
                        break;
                    }

                    // 4.1.6 C# spec: To force a value of a floating point type to the exact precision of its type, an explicit cast can be used.
                    // If this is not an identity conversion of a float with unknown precision, strip away the identity conversion.
                    if (!(explicitCastInCode && IsFloatPointExpressionOfUnknownPrecision(rewrittenOperand)))
                    {
                        return rewrittenOperand;
                    }

                    break;

                case ConversionKind.ExplicitUserDefined:
                case ConversionKind.ImplicitUserDefined:
                    return RewriteUserDefinedConversion(
                        syntax: syntax,
                        rewrittenOperand: rewrittenOperand,
                        method: symbolOpt,
                        rewrittenType: rewrittenType,
                        conversionKind: conversionKind);

                case ConversionKind.IntPtr:
                    return RewriteIntPtrConversion(oldNode, syntax, rewrittenOperand, conversionKind, symbolOpt, @checked,
                        explicitCastInCode, isExtensionMethod, isArrayIndex, constantValueOpt, rewrittenType);

                case ConversionKind.ImplicitNullable:
                case ConversionKind.ExplicitNullable:
                    return RewriteNullableConversion(
                        syntax: syntax,
                        rewrittenOperand: rewrittenOperand,
                        conversionKind: conversionKind,
                        @checked: @checked,
                        explicitCastInCode: explicitCastInCode,
                        rewrittenType: rewrittenType);

                case ConversionKind.Boxing:

                    if (!inExpressionLambda)
                    {
                        // We can perform some optimizations if we have a nullable value type
                        // as the operand and we know its nullability:

                        // * (object)new int?() is the same as (object)null
                        // * (object)new int?(123) is the same as (object)123

                        if (NullableNeverHasValue(rewrittenOperand))
                        {
                            return new BoundDefaultOperator(syntax, rewrittenType);
                        }

                        BoundExpression nullableValue = NullableAlwaysHasValue(rewrittenOperand);
                        if (nullableValue != null)
                        {
                            // Recurse, eliminating the unnecessary ctor.
                            return MakeConversion(oldNode, syntax, nullableValue, conversionKind,
                                symbolOpt, @checked, explicitCastInCode, isExtensionMethod, isArrayIndex,
                                constantValueOpt, rewrittenType);
                        }
                    }
                    break;

//.........这里部分代码省略.........

        /// <remarks>
        /// This method is intended for passes other than the LocalRewriter.
        /// Use MakeConversion helper method in the LocalRewriter instead,
        /// it generates a synthesized conversion in its lowered form.
        /// </remarks>
        public static BoundConversion SynthesizedNonUserDefined(CSharpSyntaxNode syntax, BoundExpression operand, ConversionKind kind, TypeSymbol type, ConstantValue constantValueOpt = null)
        {
            // We need more information than just the conversion kind for creating a synthesized user defined conversion.
            Debug.Assert(!kind.IsUserDefinedConversion(), "Use the BoundConversion.Synthesized overload that takes a 'Conversion' parameter for generating synthesized user defined conversions.");

            return new BoundConversion(
                syntax,
                operand,
                kind,
                resultKind: LookupResultKind.Viable, //not used
                isBaseConversion: false,
                symbolOpt: null,
                @checked: false,
                explicitCastInCode: false,
                isExtensionMethod: false,
                isArrayIndex: false,
                constantValueOpt: constantValueOpt,
                type: type)
            { WasCompilerGenerated = true };
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="binding"></param>
        /// <param name="expr"></param>
        /// <param name="ept1">expr -> TypeInParent</param>
        /// <param name="ept2">Type(expr) -> TypeInParent</param>
        private void ConversionTestHelper(SemanticModel semanticModel, ExpressionSyntax expr, ConversionKind ept1, ConversionKind ept2)
        {
            var info = semanticModel.GetTypeInfo(expr);
            Assert.NotNull(info);
            Assert.NotNull(info.ConvertedType);
            var conv = semanticModel.GetConversion(expr);

            // NOT expect NoConversion
            Conversion act1 = semanticModel.ClassifyConversion(expr, (TypeSymbol)info.ConvertedType);
            Assert.Equal(ept1, act1.Kind);
            ValidateConversion(act1, ept1);
            ValidateConversion(act1, conv.Kind);

            if (ept2 == ConversionKind.NoConversion)
            {
                Assert.Null(info.Type);
            }
            else
            {
                Assert.NotNull(info.Type);
                var act2 = semanticModel.Compilation.ClassifyConversion(info.Type, info.ConvertedType);
                Assert.Equal(ept2, act2.Kind);
                ValidateConversion(act2, ept2);
            }
        }

        public void ClassifyConversionBuiltInNumeric()
        {
            const ConversionKind ID = ConversionKind.Identity;
            const ConversionKind IN = ConversionKind.ImplicitNumeric;
            const ConversionKind XN = ConversionKind.ExplicitNumeric;

            var types = new[] { "sbyte", "byte", "short", "ushort", "int", "uint", "long", "ulong", "char", "float", "double", "decimal" };
            var conversions = new ConversionKind[,] {
                // to     sb  b  s  us i ui  l ul  c  f  d  m
                // from
                /* sb */ { ID, XN, IN, XN, IN, XN, IN, XN, XN, IN, IN, IN },
                /*  b */ { XN, ID, IN, IN, IN, IN, IN, IN, XN, IN, IN, IN },
                /*  s */ { XN, XN, ID, XN, IN, XN, IN, XN, XN, IN, IN, IN },
                /* us */ { XN, XN, XN, ID, IN, IN, IN, IN, XN, IN, IN, IN },
                /*  i */ { XN, XN, XN, XN, ID, XN, IN, XN, XN, IN, IN, IN },
                /* ui */ { XN, XN, XN, XN, XN, ID, IN, IN, XN, IN, IN, IN },
                /*  l */ { XN, XN, XN, XN, XN, XN, ID, XN, XN, IN, IN, IN },
                /* ul */ { XN, XN, XN, XN, XN, XN, XN, ID, XN, IN, IN, IN },
                /*  c */ { XN, XN, XN, IN, IN, IN, IN, IN, ID, IN, IN, IN },
                /*  f */ { XN, XN, XN, XN, XN, XN, XN, XN, XN, ID, IN, XN },
                /*  d */ { XN, XN, XN, XN, XN, XN, XN, XN, XN, XN, ID, XN },
                /*  m */ { XN, XN, XN, XN, XN, XN, XN, XN, XN, XN, XN, ID }
            };

            for (var from = 0; from < types.Length; from++)
            {
                for (var to = 0; to < types.Length; to++)
                {
                    TestClassifyConversionBuiltInNumeric(types[from], types[to], conversions[from, to]);
                }
            }
        }

        private void ValidateConversion(Conversion conv, ConversionKind kind)
        {
            Assert.Equal(conv.Kind, kind);

            switch (kind)
            {
                case ConversionKind.NoConversion:
                    Assert.False(conv.Exists);
                    Assert.False(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    break;
                case ConversionKind.Identity:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsIdentity);
                    break;
                case ConversionKind.ImplicitNumeric:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsNumeric);
                    break;
                case ConversionKind.ImplicitEnumeration:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsEnumeration);
                    break;
                case ConversionKind.ImplicitNullable:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsNullable);
                    break;
                case ConversionKind.NullLiteral:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsNullLiteral);
                    break;
                case ConversionKind.ImplicitReference:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsReference);
                    break;
                case ConversionKind.Boxing:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsBoxing);
                    break;
                case ConversionKind.ImplicitDynamic:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsDynamic);
                    break;
                case ConversionKind.ExplicitDynamic:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsExplicit);
                    Assert.False(conv.IsImplicit);
                    Assert.True(conv.IsDynamic);
                    break;
                case ConversionKind.ImplicitConstant:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsConstantExpression);
                    break;
                case ConversionKind.ImplicitUserDefined:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsUserDefined);
                    break;
                case ConversionKind.AnonymousFunction:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsAnonymousFunction);
                    break;
                case ConversionKind.MethodGroup:
                    Assert.True(conv.Exists);
                    Assert.True(conv.IsImplicit);
                    Assert.False(conv.IsExplicit);
                    Assert.True(conv.IsMethodGroup);
                    break;
                case ConversionKind.ExplicitNumeric:
                    Assert.True(conv.Exists);
                    Assert.False(conv.IsImplicit);
                    Assert.True(conv.IsExplicit);
                    Assert.True(conv.IsNumeric);
                    break;
                case ConversionKind.ExplicitEnumeration:
                    Assert.True(conv.Exists);
                    Assert.False(conv.IsImplicit);
                    Assert.True(conv.IsExplicit);
                    Assert.True(conv.IsEnumeration);
//.........这里部分代码省略.........

        private void TestClassifyConversionBuiltInNumeric(string from, string to, ConversionKind ck)
        {
            const string template = @"
class C
{{
    static void Foo({1} v) {{ }}

    static void Main() {{ {0} v = default({0}); Foo({2}v); }}
}}
";

            var isExplicitConversion = ck == ConversionKind.ExplicitNumeric;
            var source = string.Format(template, from, to, isExplicitConversion ? "(" + to + ")" : "");
            var tree = Parse(source);
            var comp = CreateCompilationWithMscorlib(tree);
            comp.VerifyDiagnostics();

            var model = comp.GetSemanticModel(tree);
            var c = (TypeDeclarationSyntax)tree.GetCompilationUnitRoot().Members[0];
            var main = (MethodDeclarationSyntax)c.Members[1];
            var call = (InvocationExpressionSyntax)((ExpressionStatementSyntax)main.Body.Statements[1]).Expression;
            var arg = call.ArgumentList.Arguments[0].Expression;

            if (isExplicitConversion)
            {
                ConversionTestHelper(model, ((CastExpressionSyntax)arg).Expression, model.GetTypeInfo(arg).ConvertedType, ck);
            }
            else
            {
                ConversionTestHelper(model, arg, ck, ck);
            }
        }

        private BoundExpression RewriteUserDefinedConversion(
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            MethodSymbol method,
            TypeSymbol rewrittenType,
            ConversionKind conversionKind)
        {
            Debug.Assert((object)method != null && !method.ReturnsVoid && method.ParameterCount == 1);
            if (rewrittenOperand.Type != method.ParameterTypes[0])
            {
                Debug.Assert(rewrittenOperand.Type.IsNullableType());
                Debug.Assert(rewrittenOperand.Type.GetNullableUnderlyingType() == method.ParameterTypes[0]);
                return RewriteLiftedUserDefinedConversion(syntax, rewrittenOperand, method, rewrittenType, conversionKind);
            }

            var conv = new Conversion(ConversionKind.ExplicitUserDefined, method, false);
            Debug.Assert(conv.IsValid);
            BoundExpression result =
                inExpressionLambda
                ? BoundConversion.Synthesized(syntax, rewrittenOperand, MakeConversion(syntax, conv, rewrittenOperand.Type, rewrittenType), false, true, default(ConstantValue), rewrittenType)
                : (BoundExpression)BoundCall.Synthesized(syntax, null, method, rewrittenOperand);
            Debug.Assert(result.Type == rewrittenType);
            return result;
        }

        private BoundExpression RewriteLiftedUserDefinedConversion(
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            MethodSymbol method,
            TypeSymbol rewrittenType,
            ConversionKind conversionKind)
        {
            if (inExpressionLambda)
            {
                Conversion conv = MakeConversion(syntax, new Conversion(conversionKind, method, false), rewrittenOperand.Type, rewrittenType);
                return BoundConversion.Synthesized(syntax, rewrittenOperand, conv, false, true, default(ConstantValue), rewrittenType);
            }

            // DELIBERATE SPEC VIOLATION: 
            // The native compiler allows for a "lifted" conversion even when the return type of the conversion
            // not a non-nullable value type. For example, if we have a conversion from struct S to string,
            // then a "lifted" conversion from S? to string is considered by the native compiler to exist,
            // with the semantics of "s.HasValue ? (string)s.Value : (string)null".  The Roslyn compiler
            // perpetuates this error for the sake of backwards compatibility.

            Debug.Assert((object)rewrittenType != null);
            Debug.Assert(rewrittenOperand.Type.IsNullableType());

            BoundExpression optimized = OptimizeLiftedUserDefinedConversion(syntax, rewrittenOperand, conversionKind, method, rewrittenType);
            if (optimized != null)
            {
                return optimized;
            }

            // We have no optimizations we can perform. If the return type of the 
            // conversion method is a non-nullable value type R then we lower this as:
            //
            // temp = operand
            // temp.HasValue ? new R?(op_Whatever(temp.GetValueOrDefault())) : default(R?)
            //
            // Otherwise, if the return type of the conversion is a nullable value type, reference type
            // or pointer type P, then we lower this as:
            //
            // temp = operand
            // temp.HasValue ? op_Whatever(temp.GetValueOrDefault()) : default(P)

            BoundAssignmentOperator tempAssignment;
            BoundLocal boundTemp = factory.StoreToTemp(rewrittenOperand, out tempAssignment);
            MethodSymbol get_HasValue = GetNullableMethod(syntax, boundTemp.Type, SpecialMember.System_Nullable_T_get_HasValue);
            MethodSymbol getValueOrDefault = GetNullableMethod(syntax, boundTemp.Type, SpecialMember.System_Nullable_T_GetValueOrDefault);

            // temp.HasValue
            BoundExpression condition = BoundCall.Synthesized(syntax, boundTemp, get_HasValue);

            // temp.GetValueOrDefault()
            BoundCall callGetValueOrDefault = BoundCall.Synthesized(syntax, boundTemp, getValueOrDefault);

            // op_Whatever(temp.GetValueOrDefault())
            BoundCall userDefinedCall = BoundCall.Synthesized(syntax, null, method, callGetValueOrDefault);

            // new R?(op_Whatever(temp.GetValueOrDefault())
            BoundExpression consequence = MakeLiftedUserDefinedConversionConsequence(userDefinedCall, rewrittenType);

            // default(R?)
            BoundExpression alternative = new BoundDefaultOperator(syntax, rewrittenType);

            // temp.HasValue ? new R?(op_Whatever(temp.GetValueOrDefault())) : default(R?)
            BoundExpression conditionalExpression = RewriteConditionalOperator(
                syntax: syntax,
                rewrittenCondition: condition,
                rewrittenConsequence: consequence,
                rewrittenAlternative: alternative,
                constantValueOpt: null,
                rewrittenType: rewrittenType);

            // temp = operand
            // temp.HasValue ? new R?(op_Whatever(temp.GetValueOrDefault())) : default(R?)
            return new BoundSequence(
                syntax: syntax,
                locals: ImmutableArray.Create(boundTemp.LocalSymbol),
                sideEffects: ImmutableArray.Create<BoundExpression>(tempAssignment),
                value: conditionalExpression,
                type: rewrittenType);
        }

        private BoundExpression RewriteNullableConversion(
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            ConversionKind conversionKind,
            bool @checked,
            bool explicitCastInCode,
            TypeSymbol rewrittenType)
        {
            Debug.Assert((object)rewrittenType != null);

            if (inExpressionLambda)
            {
                return RewriteLiftedConversionInExpressionTree(syntax, rewrittenOperand, conversionKind, @checked, explicitCastInCode, rewrittenType);
            }

            TypeSymbol rewrittenOperandType = rewrittenOperand.Type;
            Debug.Assert(rewrittenType.IsNullableType() || rewrittenOperandType.IsNullableType());

            if (rewrittenOperandType.IsNullableType() && rewrittenType.IsNullableType())
            {
                return RewriteFullyLiftedBuiltInConversion(syntax, rewrittenOperand, conversionKind, @checked, rewrittenType);
            }
            else if (rewrittenType.IsNullableType())
            {
                // SPEC: If the nullable conversion is from S to T?, the conversion is 
                // SPEC: evaluated as the underlying conversion from S to T followed
                // SPEC: by a wrapping from T to T?.

                BoundExpression rewrittenConversion = MakeConversion(rewrittenOperand, rewrittenType.GetNullableUnderlyingType(), @checked);
                MethodSymbol ctor = GetNullableMethod(syntax, rewrittenType, SpecialMember.System_Nullable_T__ctor);
                return new BoundObjectCreationExpression(syntax, ctor, rewrittenConversion);
            }
            else
            {
                // SPEC: if the nullable conversion is from S? to T, the conversion is
                // SPEC: evaluated as an unwrapping from S? to S followed by the underlying
                // SPEC: conversion from S to T.

                // We can do a simple optimization here if we know that the source is never null:

                BoundExpression nonNullValue = NullableAlwaysHasValue(rewrittenOperand);
                if (nonNullValue != null)
                {
                    return MakeConversion(nonNullValue, rewrittenType, @checked);
                }

                // (If the source is known to be null then we need to keep the call to get Value 
                // in place so that it throws at runtime.)

                MethodSymbol get_Value = GetNullableMethod(syntax, rewrittenOperandType, SpecialMember.System_Nullable_T_get_Value);
                return MakeConversion(BoundCall.Synthesized(syntax, rewrittenOperand, get_Value), rewrittenType, @checked);
            }
        }

        private BoundExpression RewriteIntPtrConversion(
            BoundConversion oldNode,
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            ConversionKind conversionKind,
            MethodSymbol symbolOpt,
            bool @checked,
            bool explicitCastInCode,
            bool isExtensionMethod,
            bool isArrayIndex,
            ConstantValue constantValueOpt,
            TypeSymbol rewrittenType)
        {
            Debug.Assert(rewrittenOperand != null);
            Debug.Assert((object)rewrittenType != null);

            TypeSymbol source = rewrittenOperand.Type;
            TypeSymbol target = rewrittenType;

            TypeSymbol t0 = target.StrippedType();
            TypeSymbol s0 = source.StrippedType();

            if (t0 != target && s0 != source)
            {
                // UNDONE: RewriteLiftedIntPtrConversion

                return oldNode != null ?
                    oldNode.Update(
                        rewrittenOperand,
                        conversionKind,
                        oldNode.ResultKind,
                        isBaseConversion: oldNode.IsBaseConversion,
                        symbolOpt: symbolOpt,
                        @checked: @checked,
                        explicitCastInCode: explicitCastInCode,
                        isExtensionMethod: isExtensionMethod,
                        isArrayIndex: isArrayIndex,
                        constantValueOpt: constantValueOpt,
                        type: rewrittenType) :
                    new BoundConversion(
                        syntax,
                        rewrittenOperand,
                        conversionKind,
                        LookupResultKind.Viable,
                        isBaseConversion: false,
                        symbolOpt: symbolOpt,
                        @checked: @checked,
                        explicitCastInCode: explicitCastInCode,
                        isExtensionMethod: isExtensionMethod,
                        isArrayIndex: isArrayIndex,
                        constantValueOpt: constantValueOpt,
                        type: rewrittenType);
            }

            SpecialMember member = GetIntPtrConversionMethod(source: rewrittenOperand.Type, target: rewrittenType);
            MethodSymbol method = GetSpecialTypeMethod(syntax, member);
            Debug.Assert(!method.ReturnsVoid);
            Debug.Assert(method.ParameterCount == 1);

            rewrittenOperand = MakeConversion(rewrittenOperand, method.ParameterTypes[0], @checked);

            var returnType = method.ReturnType;
            Debug.Assert((object)returnType != null);

            if (inExpressionLambda)
            {
                return BoundConversion.Synthesized(syntax, rewrittenOperand, new Conversion(conversionKind, method, false), @checked, explicitCastInCode, constantValueOpt, rewrittenType);
            }

            var rewrittenCall =
                inExpressionLambda && oldNode != null
                ? new BoundConversion(syntax, rewrittenOperand, new Conversion(conversionKind, method, false), @checked, explicitCastInCode, constantValueOpt, returnType)
                : MakeCall(
                    syntax: syntax,
                    rewrittenReceiver: null,
                    method: method,
                    rewrittenArguments: ImmutableArray.Create<BoundExpression>(rewrittenOperand),
                    type: returnType);

            return MakeConversion(rewrittenCall, rewrittenType, @checked);
        }

        private BoundExpression RewriteLiftedConversionInExpressionTree(
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            ConversionKind conversionKind,
            bool @checked,
            bool explicitCastInCode,
            TypeSymbol rewrittenType)
        {
            Debug.Assert((object)rewrittenType != null);
            TypeSymbol rewrittenOperandType = rewrittenOperand.Type;
            Debug.Assert(rewrittenType.IsNullableType() || rewrittenOperandType.IsNullableType());

            TypeSymbol typeFrom = rewrittenOperandType.StrippedType();
            TypeSymbol typeTo = rewrittenType.StrippedType();
            if (typeFrom != typeTo && (typeFrom.SpecialType == SpecialType.System_Decimal || typeTo.SpecialType == SpecialType.System_Decimal))
            {
                // take special care if the underlying conversion is a decimal conversion
                TypeSymbol typeFromUnderlying = typeFrom;
                TypeSymbol typeToUnderlying = typeTo;

                // They can't both be enums, since one of them is decimal.
                if (typeFrom.IsEnumType())
                {
                    typeFromUnderlying = typeFrom.GetEnumUnderlyingType();

                    // NOTE: Dev10 converts enum? to underlying?, rather than directly to underlying.
                    rewrittenOperandType = rewrittenOperandType.IsNullableType() ? ((NamedTypeSymbol)rewrittenOperandType.OriginalDefinition).Construct(typeFromUnderlying) : typeFromUnderlying;
                    rewrittenOperand = BoundConversion.SynthesizedNonUserDefined(syntax, rewrittenOperand, ConversionKind.ImplicitEnumeration, rewrittenOperandType);
                }
                else if (typeTo.IsEnumType())
                {
                    typeToUnderlying = typeTo.GetEnumUnderlyingType();
                }

                var method = (MethodSymbol)this.compilation.Assembly.GetSpecialTypeMember(DecimalConversionMethod(typeFromUnderlying, typeToUnderlying));
                conversionKind = conversionKind.IsImplicitConversion() ? ConversionKind.ImplicitUserDefined : ConversionKind.ExplicitUserDefined;
                var result = new BoundConversion(syntax, rewrittenOperand, new Conversion(conversionKind, method, false), @checked, explicitCastInCode, default(ConstantValue), rewrittenType);
                return result;
            }
            else
            {
                return new BoundConversion(syntax, rewrittenOperand, new Conversion(conversionKind), @checked, explicitCastInCode, default(ConstantValue), rewrittenType);
            }
        }

 public BoundExpression Convert(TypeSymbol type, BoundExpression arg, ConversionKind conversionKind, bool isChecked = false)
 {
     return new BoundConversion(
         Syntax,
         arg,
         conversionKind,
         LookupResultKind.Viable,
         isBaseConversion: false,
         symbolOpt: null,
         @checked: isChecked,
         explicitCastInCode: true,
         isExtensionMethod: false,
         isArrayIndex: false,
         constantValueOpt: null,
         type: type)
         { WasCompilerGenerated = true };
 }

        private BoundExpression RewriteFullyLiftedBuiltInConversion(
            CSharpSyntaxNode syntax,
            BoundExpression operand,
            ConversionKind kind,
            bool @checked,
            TypeSymbol type)
        {
            // SPEC: If the nullable conversion is from S? to T?:
            // SPEC: * If the source HasValue property is false the result
            // SPEC:   is a null value of type T?.
            // SPEC: * Otherwise the conversion is evaluated as an unwrapping
            // SPEC:   from S? to S, followed by the underlying conversion from
            // SPEC:   S to T, followed by a wrapping from T to T?

            BoundExpression optimized = OptimizeLiftedBuiltInConversion(syntax, operand, kind, @checked, type);
            if (optimized != null)
            {
                return optimized;
            }

            // We are unable to optimize the conversion. "(T?)s" is generated as:
            // S? temp = s;
            // temp.HasValue ? new T?((T)temp.GetValueOrDefault()) : default(T?)

            BoundAssignmentOperator tempAssignment;
            var boundTemp = factory.StoreToTemp(operand, out tempAssignment);
            MethodSymbol get_HasValue = GetNullableMethod(syntax, boundTemp.Type, SpecialMember.System_Nullable_T_get_HasValue);
            MethodSymbol getValueOrDefault = GetNullableMethod(syntax, boundTemp.Type, SpecialMember.System_Nullable_T_GetValueOrDefault);
            BoundExpression condition = BoundCall.Synthesized(syntax, boundTemp, get_HasValue);
            BoundExpression consequence = new BoundObjectCreationExpression(
                syntax,
                GetNullableMethod(syntax, type, SpecialMember.System_Nullable_T__ctor),
                MakeConversion(
                    BoundCall.Synthesized(syntax, boundTemp, getValueOrDefault),
                    type.GetNullableUnderlyingType(),
                    @checked));
            BoundExpression alternative = new BoundDefaultOperator(syntax, null, type);
            BoundExpression conditionalExpression = RewriteConditionalOperator(
                syntax: syntax,
                rewrittenCondition: condition,
                rewrittenConsequence: consequence,
                rewrittenAlternative: alternative,
                constantValueOpt: null,
                rewrittenType: type);

            return new BoundSequence(
                syntax: syntax,
                locals: ImmutableArray.Create(boundTemp.LocalSymbol),
                sideEffects: ImmutableArray.Create<BoundExpression>(tempAssignment),
                value: conditionalExpression,
                type: type);
        }

        private BoundExpression OptimizeLiftedUserDefinedConversion(
            CSharpSyntaxNode syntax,
            BoundExpression operand,
            ConversionKind kind,
            MethodSymbol method,
            TypeSymbol type)
        {
            // We begin with some optimizations: if the converted expression is known to always be null
            // then we can skip the whole thing and simply return the alternative:

            if (NullableNeverHasValue(operand))
            {
                return new BoundDefaultOperator(syntax, type);