C# TypeSymbol.IsNullableType方法代码示例

        private BoundExpression MakeAsOperator(
            BoundAsOperator oldNode,
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            BoundTypeExpression rewrittenTargetType,
            Conversion conversion,
            TypeSymbol rewrittenType)
        {
            // TODO: Handle dynamic operand type and target type
            Debug.Assert(rewrittenTargetType.Type.Equals(rewrittenType));

            // target type cannot be a non-nullable value type
            Debug.Assert(!rewrittenType.IsValueType || rewrittenType.IsNullableType());

            if (!_inExpressionLambda)
            {
                ConstantValue constantValue = Binder.GetAsOperatorConstantResult(rewrittenOperand.Type, rewrittenType, conversion.Kind, rewrittenOperand.ConstantValue);

                if (constantValue != null)
                {
                    Debug.Assert(constantValue.IsNull);
                    BoundExpression result = rewrittenType.IsNullableType() ? new BoundDefaultOperator(syntax, rewrittenType) : MakeLiteral(syntax, constantValue, rewrittenType);

                    if (rewrittenOperand.ConstantValue != null)
                    {
                        // No need to preserve any side-effects from the operand. 
                        // We also can keep the "constant" notion of the result, which
                        // enables some optimizations down the road.
                        return result;
                    }

                    return new BoundSequence(
                        syntax: syntax,
                        locals: ImmutableArray<LocalSymbol>.Empty,
                        sideEffects: ImmutableArray.Create<BoundExpression>(rewrittenOperand),
                        value: result,
                        type: rewrittenType);
                }

                if (conversion.IsImplicit)
                {
                    // Operand with bound implicit conversion to target type.
                    // We don't need a runtime check, generate a conversion for the operand instead.
                    return MakeConversionNode(syntax, rewrittenOperand, conversion, rewrittenType, @checked: false);
                }
            }

            return oldNode.Update(rewrittenOperand, rewrittenTargetType, conversion, rewrittenType);
        }

        private BoundExpression MakeAsOperator(
            BoundAsOperator oldNode,
            CSharpSyntaxNode syntax,
            BoundExpression rewrittenOperand,
            BoundTypeExpression rewrittenTargetType,
            Conversion conversion,
            TypeSymbol rewrittenType)
        {
            // TODO: Handle dynamic operand type and target type
            Debug.Assert(rewrittenTargetType.Type.Equals(rewrittenType));

            // target type cannot be a non-nullable value type
            Debug.Assert(!rewrittenType.IsValueType || rewrittenType.IsNullableType());

            if (!inExpressionLambda)
            {
                ConstantValue constantValue = Binder.GetAsOperatorConstantResult(rewrittenOperand.Type, rewrittenType, conversion.Kind, rewrittenOperand.ConstantValue);
                Debug.Assert(constantValue == null || constantValue.IsNull);

                if (conversion.IsImplicit)
                {
                    // Operand with bound implicit conversion to target type.
                    // We don't need a runtime check, generate a conversion for the operand instead.
                    return MakeConversion(syntax, rewrittenOperand, conversion, rewrittenType, @checked: false, constantValueOpt: constantValue);
                }
                else if (constantValue != null)
                {
                    return new BoundSequence(
                        syntax: syntax,
                        locals: ImmutableArray<LocalSymbol>.Empty,
                        sideEffects: ImmutableArray.Create<BoundExpression>(rewrittenOperand),
                        value: MakeLiteral(syntax, constantValue, rewrittenType),
                        type: rewrittenType);
                }
            }

            return oldNode.Update(rewrittenOperand, rewrittenTargetType, conversion, rewrittenType);
        }

        // See TypeBind::CheckSingleConstraint.
        private static bool CheckConstraints(
            Symbol containingSymbol,
            ConversionsBase conversions,
            TypeMap substitution,
            TypeParameterSymbol typeParameter,
            TypeSymbol typeArgument,
            Compilation currentCompilation,
            ArrayBuilder<TypeParameterDiagnosticInfo> diagnosticsBuilder,
            ref ArrayBuilder<TypeParameterDiagnosticInfo> useSiteDiagnosticsBuilder,
            HashSet<TypeParameterSymbol> ignoreTypeConstraintsDependentOnTypeParametersOpt)
        {
            Debug.Assert(substitution != null);
            // The type parameters must be original definitions of type parameters from the containing symbol.
            Debug.Assert(ReferenceEquals(typeParameter.ContainingSymbol, containingSymbol.OriginalDefinition));

            if (typeArgument.IsErrorType())
            {
                return true;
            }

            if (typeArgument.IsPointerType() || typeArgument.IsRestrictedType() || typeArgument.SpecialType == SpecialType.System_Void)
            {
                // "The type '{0}' may not be used as a type argument"
                diagnosticsBuilder.Add(new TypeParameterDiagnosticInfo(typeParameter, new CSDiagnosticInfo(ErrorCode.ERR_BadTypeArgument, typeArgument)));
                return false;
            }

            if (typeArgument.IsStatic)
            {
                // "'{0}': static types cannot be used as type arguments"
                diagnosticsBuilder.Add(new TypeParameterDiagnosticInfo(typeParameter, new CSDiagnosticInfo(ErrorCode.ERR_GenericArgIsStaticClass, typeArgument)));
                return false;
            }

            if (typeParameter.HasReferenceTypeConstraint && !typeArgument.IsReferenceType)
            {
                // "The type '{2}' must be a reference type in order to use it as parameter '{1}' in the generic type or method '{0}'"
                diagnosticsBuilder.Add(new TypeParameterDiagnosticInfo(typeParameter, new CSDiagnosticInfo(ErrorCode.ERR_RefConstraintNotSatisfied, containingSymbol.ConstructedFrom(), typeParameter, typeArgument)));
                return false;
            }

            if (typeParameter.HasValueTypeConstraint && !typeArgument.IsNonNullableValueType())
            {
                // "The type '{2}' must be a non-nullable value type in order to use it as parameter '{1}' in the generic type or method '{0}'"
                diagnosticsBuilder.Add(new TypeParameterDiagnosticInfo(typeParameter, new CSDiagnosticInfo(ErrorCode.ERR_ValConstraintNotSatisfied, containingSymbol.ConstructedFrom(), typeParameter, typeArgument)));
                return false;
            }

            // The type parameters for a constructed type/method are the type parameters of
            // the ConstructedFrom type/method, so the constraint types are not substituted.
            // For instance with "class C<T, U> where T : U", the type parameter for T in "C<object, int>"
            // has constraint "U", not "int". We need to substitute the constraints from the
            // original definition of the type parameters using the map from the constructed symbol.
            var constraintTypes = ArrayBuilder<TypeSymbol>.GetInstance();
            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
            substitution.SubstituteTypesDistinctWithoutModifiers(typeParameter.ConstraintTypesWithDefinitionUseSiteDiagnostics(ref useSiteDiagnostics), constraintTypes, 
                                                                 ignoreTypeConstraintsDependentOnTypeParametersOpt);

            bool hasError = false;

            foreach (var constraintType in constraintTypes)
            {
                if (SatisfiesConstraintType(conversions, typeArgument, constraintType, ref useSiteDiagnostics))
                {
                    continue;
                }

                ErrorCode errorCode;
                if (typeArgument.IsReferenceType)
                {
                    errorCode = ErrorCode.ERR_GenericConstraintNotSatisfiedRefType;
                }
                else if (typeArgument.IsNullableType())
                {
                    errorCode = constraintType.IsInterfaceType() ? ErrorCode.ERR_GenericConstraintNotSatisfiedNullableInterface : ErrorCode.ERR_GenericConstraintNotSatisfiedNullableEnum;
                }
                else if (typeArgument.TypeKind == TypeKind.TypeParameter)
                {
                    errorCode = ErrorCode.ERR_GenericConstraintNotSatisfiedTyVar;
                }
                else
                {
                    errorCode = ErrorCode.ERR_GenericConstraintNotSatisfiedValType;
                }

                SymbolDistinguisher distinguisher = new SymbolDistinguisher(currentCompilation, constraintType, typeArgument);
                diagnosticsBuilder.Add(new TypeParameterDiagnosticInfo(typeParameter, new CSDiagnosticInfo(errorCode, containingSymbol.ConstructedFrom(), distinguisher.First, typeParameter, distinguisher.Second)));
                hasError = true;
            }

            if (AppendUseSiteDiagnostics(useSiteDiagnostics, typeParameter, ref useSiteDiagnosticsBuilder))
            {
                hasError = true;
            }

            constraintTypes.Free();

            // Check the constructor constraint.
            if (typeParameter.HasConstructorConstraint && !SatisfiesConstructorConstraint(typeArgument))
//.........这里部分代码省略.........

        private BoundExpression ConvertCaseExpression(TypeSymbol switchGoverningType, CSharpSyntaxNode node, BoundExpression caseExpression, Binder sectionBinder, ref ConstantValue constantValueOpt, DiagnosticBag diagnostics, bool isGotoCaseExpr = false)
        {
            BoundExpression convertedCaseExpression;
            if (!isGotoCaseExpr)
            {
                // NOTE: This will allow user-defined conversions, even though they're not allowed here.  This is acceptable
                // because the result of a user-defined conversion does not have a ConstantValue and we'll report a diagnostic
                // to that effect below (same error code as Dev10).
                convertedCaseExpression = sectionBinder.GenerateConversionForAssignment(switchGoverningType, caseExpression, diagnostics);
            }
            else
            {
                // SPEC VIOLATION for Dev10 COMPATIBILITY:

                // Dev10 compiler violates the SPEC comment below:
                //      "if the constant-expression is not implicitly convertible (§6.1) to 
                //      the governing type of the nearest enclosing switch statement, 
                //      a compile-time error occurs"

                // If there is no implicit conversion from gotoCaseExpression to switchGoverningType,
                // but there exists an explicit conversion, Dev10 compiler generates a warning "WRN_GotoCaseShouldConvert"
                // instead of an error. See test "CS0469_NoImplicitConversionWarning".

                // CONSIDER: Should we introduce a breaking change and violate Dev10 compatibility and follow the spec?

                HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                Conversion conversion = sectionBinder.Conversions.ClassifyConversionFromExpression(caseExpression, switchGoverningType, ref useSiteDiagnostics);
                diagnostics.Add(node, useSiteDiagnostics);
                if (!conversion.IsValid)
                {
                    GenerateImplicitConversionError(diagnostics, node, conversion, caseExpression, switchGoverningType);
                }
                else if (!conversion.IsImplicit)
                {
                    diagnostics.Add(ErrorCode.WRN_GotoCaseShouldConvert, node.Location, switchGoverningType);
                }

                convertedCaseExpression = sectionBinder.CreateConversion(caseExpression, conversion, switchGoverningType, diagnostics);
            }

            if (switchGoverningType.IsNullableType()
                && convertedCaseExpression.Kind == BoundKind.Conversion
                // Null is a special case here because we want to compare null to the Nullable<T> itself, not to the underlying type.
                && (convertedCaseExpression.ConstantValue == null || !convertedCaseExpression.ConstantValue.IsNull))
            {
                var operand = ((BoundConversion)convertedCaseExpression).Operand;

                // We are not intested in the diagnostic that get created here
                var diagnosticBag = DiagnosticBag.GetInstance();
                constantValueOpt = sectionBinder.CreateConversion(operand, switchGoverningType.GetNullableUnderlyingType(), diagnosticBag).ConstantValue;
                diagnosticBag.Free();
            }
            else
            {
                constantValueOpt = convertedCaseExpression.ConstantValue;
            }

            return convertedCaseExpression;
        }

        private TypeSymbol TypeOrUnderlyingType(TypeSymbol type)
        {
            if (type.IsNullableType())
            {
                return type.GetNullableUnderlyingType();
            }

            return type;
        }

        private void AddUserDefinedConversionsToExplicitCandidateSet(
            BoundExpression sourceExpression,
            TypeSymbol source,
            TypeSymbol target,
            ArrayBuilder<UserDefinedConversionAnalysis> u,
            NamedTypeSymbol declaringType,
            string operatorName,
            ref HashSet<DiagnosticInfo> useSiteDiagnostics)
        {
            Debug.Assert(sourceExpression != null || (object)source != null);
            Debug.Assert((object)target != null);
            Debug.Assert(u != null);
            Debug.Assert((object)declaringType != null);
            Debug.Assert(operatorName != null);

            // SPEC: Find the set of applicable user-defined and lifted conversion operators, U.
            // SPEC: The set consists of the user-defined and lifted implicit or explicit 
            // SPEC: conversion operators declared by the classes and structs in D that convert 
            // SPEC: from a type encompassing E or encompassed by S (if it exists) to a type
            // SPEC: encompassing or encompassed by T. 

            // DELIBERATE SPEC VIOLATION:
            //
            // The spec here essentially says that we add an applicable "regular" conversion and 
            // an applicable lifted conversion, if there is one, to the candidate set, and then
            // let them duke it out to determine which one is "best".
            //
            // This is not at all what the native compiler does, and attempting to implement
            // the specification, or slight variations on it, produces too many backwards-compatibility
            // breaking changes.
            //
            // The native compiler deviates from the specification in two major ways here.
            // First, it does not add *both* the regular and lifted forms to the candidate set.
            // Second, the way it characterizes a "lifted" form is very, very different from
            // how the specification characterizes a lifted form. 
            //
            // An operation, in this case, X-->Y, is properly said to be "lifted" to X?-->Y? via
            // the rule that X?-->Y? matches the behavior of X-->Y for non-null X, and converts
            // null X to null Y otherwise.
            //
            // The native compiler, by contrast, takes the existing operator and "lifts" either
            // the operator's parameter type or the operator's return type to nullable. For
            // example, a conversion from X?-->Y would be "lifted" to X?-->Y? by making the
            // conversion from X? to Y, and then from Y to Y?.  No "lifting" semantics
            // are imposed; we do not check to see if the X? is null. This operator is not
            // actually "lifted" at all; rather, an implicit conversion is applied to the 
            // output. **The native compiler considers the result type Y? of that standard implicit
            // conversion to be the result type of the "lifted" conversion**, rather than
            // properly considering Y to be the result type of the conversion for the purposes 
            // of computing the best output type.
            //
            // Moreover: the native compiler actually *does* implement nullable lifting semantics
            // in the case where the input type of the user-defined conversion is a non-nullable
            // value type and the output type is a nullable value type **or pointer type, or 
            // reference type**. This is an enormous departure from the specification; the
            // native compiler will take a user-defined conversion from X-->Y? or X-->C and "lift"
            // it to a conversion from X?-->Y? or X?-->C that has nullable semantics.
            // 
            // This is quite confusing. In this code we will classify the conversion as either
            // "normal" or "lifted" on the basis of *whether or not special lifting semantics
            // are to be applied*. That is, whether or not a later rewriting pass is going to
            // need to insert a check to see if the source expression is null, and decide
            // whether or not to call the underlying unlifted conversion or produce a null
            // value without calling the unlifted conversion.
            // DELIBERATE SPEC VIOLATION: See the comment regarding bug 17021 in 
            // UserDefinedImplicitConversions.cs.

            if ((object)source != null && source.IsInterfaceType() || target.IsInterfaceType())
            {
                return;
            }

            foreach (MethodSymbol op in declaringType.GetOperators(operatorName))
            {
                // We might have a bad operator and be in an error recovery situation. Ignore it.
                if (op.ReturnsVoid || op.ParameterCount != 1 || op.ReturnType.TypeKind == TypeKind.Error)
                {
                    continue;
                }

                TypeSymbol convertsFrom = op.ParameterTypes[0];
                TypeSymbol convertsTo = op.ReturnType;
                Conversion fromConversion = EncompassingExplicitConversion(sourceExpression, source, convertsFrom, ref useSiteDiagnostics);
                Conversion toConversion = EncompassingExplicitConversion(null, convertsTo, target, ref useSiteDiagnostics);

                // We accept candidates for which the parameter type encompasses the *underlying* source type.
                if (!fromConversion.Exists &&
                    (object)source != null &&
                    source.IsNullableType() &&
                    EncompassingExplicitConversion(null, source.GetNullableUnderlyingType(), convertsFrom, ref useSiteDiagnostics).Exists)
                {
                    fromConversion = ClassifyConversion(source, convertsFrom, ref useSiteDiagnostics, builtinOnly: true);
                }

                // As in dev11 (and the revised spec), we also accept candidates for which the return type is encompassed by the *stripped* target type.
                if (!toConversion.Exists &&
                    (object)target != null &&
                    target.IsNullableType() &&
                    EncompassingExplicitConversion(null, convertsTo, target.GetNullableUnderlyingType(), ref useSiteDiagnostics).Exists)
                {
//.........这里部分代码省略.........

        private static ConversionKind ClassifyNullLiteralConversion(BoundExpression source, TypeSymbol destination)
        {
            Debug.Assert((object)source != null);
            Debug.Assert((object)destination != null);

            if (!source.IsLiteralNull())
            {
                return ConversionKind.NoConversion;
            }

            // SPEC: An implicit conversion exists from the null literal to any nullable type. 
            if (destination.IsNullableType())
            {
                // The spec defines a "null literal conversion" specifically as a conversion from
                // null to nullable type.
                return ConversionKind.NullLiteral;
            }

            // SPEC: An implicit conversion exists from the null literal to any reference type. 
            // SPEC: An implicit conversion exists from the null literal to type parameter T, 
            // SPEC: provided T is known to be a reference type. [...] The conversion [is] classified 
            // SPEC: as implicit reference conversion. 

            if (destination.IsReferenceType)
            {
                return ConversionKind.ImplicitReference;
            }

            // SPEC: The set of implicit conversions is extended to include...
            // SPEC: ... from the null literal to any pointer type.

            if (destination is PointerTypeSymbol)
            {
                return ConversionKind.NullToPointer;
            }

            return ConversionKind.NoConversion;
        }

        private static LiftingResult UserDefinedBinaryOperatorCanBeLifted(TypeSymbol left, TypeSymbol right, TypeSymbol result, BinaryOperatorKind kind)
        {
            // SPEC: For the binary operators + - * / % & | ^ << >> a lifted form of the
            // SPEC: operator exists if the operand and result types are all non-nullable
            // SPEC: value types. The lifted form is constructed by adding a single ?
            // SPEC: modifier to each operand and result type. 
            //
            // SPEC: For the equality operators == != a lifted form of the operator exists
            // SPEC: if the operand types are both non-nullable value types and if the 
            // SPEC: result type is bool. The lifted form is constructed by adding
            // SPEC: a single ? modifier to each operand type.
            //
            // SPEC: For the relational operators > < >= <= a lifted form of the 
            // SPEC: operator exists if the operand types are both non-nullable value
            // SPEC: types and if the result type is bool. The lifted form is 
            // SPEC: constructed by adding a single ? modifier to each operand type.

            if (!left.IsValueType ||
                left.IsNullableType() ||
                !right.IsValueType ||
                right.IsNullableType())
            {
                return LiftingResult.NotLifted;
            }

            switch (kind)
            {
                case BinaryOperatorKind.Equal:
                case BinaryOperatorKind.NotEqual:
                    // Spec violation: can't lift unless the types match.
                    // The spec doesn't require this, but dev11 does and it reduces ambiguity in some cases.
                    if (left != right) return LiftingResult.NotLifted;
                    goto case BinaryOperatorKind.GreaterThan;
                case BinaryOperatorKind.GreaterThan:
                case BinaryOperatorKind.GreaterThanOrEqual:
                case BinaryOperatorKind.LessThan:
                case BinaryOperatorKind.LessThanOrEqual:
                    return result.SpecialType == SpecialType.System_Boolean ?
                        LiftingResult.LiftOperandsButNotResult :
                        LiftingResult.NotLifted;
                default:
                    return result.IsValueType && !result.IsNullableType() ?
                        LiftingResult.LiftOperandsAndResult :
                        LiftingResult.NotLifted;
            }
        }

        private bool CheckValidPatternType(
            CSharpSyntaxNode typeSyntax,
            BoundExpression operand,
            TypeSymbol operandType,
            TypeSymbol patternType,
            bool patternTypeWasInSource,
            bool isVar,
            DiagnosticBag diagnostics)
        {
            if (operandType?.IsErrorType() == true || patternType?.IsErrorType() == true)
            {
                return false;
            }
            else if (patternType.IsNullableType() && !isVar && patternTypeWasInSource)
            {
                // It is an error to use pattern-matching with a nullable type, because you'll never get null. Use the underlying type.
                Error(diagnostics, ErrorCode.ERR_PatternNullableType, typeSyntax, patternType, patternType.GetNullableUnderlyingType());
                return true;
            }
            else if (patternType.IsStatic)
            {
                Error(diagnostics, ErrorCode.ERR_VarDeclIsStaticClass, typeSyntax, patternType);
                return true;
            }
            else if (operand != null && operandType == (object)null && !operand.HasAnyErrors)
            {
                // It is an error to use pattern-matching with a null, method group, or lambda
                Error(diagnostics, ErrorCode.ERR_BadIsPatternExpression, operand.Syntax);
                return true;
            }
            else if (!isVar)
            {
                HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                Conversion conversion =
                    operand != null
                    ? this.Conversions.ClassifyConversionFromExpression(operand, patternType, ref useSiteDiagnostics, forCast: true)
                    : this.Conversions.ClassifyConversionFromType(operandType, patternType, ref useSiteDiagnostics, forCast: true);
                diagnostics.Add(typeSyntax, useSiteDiagnostics);
                switch (conversion.Kind)
                {
                    case ConversionKind.Boxing:
                    case ConversionKind.ExplicitNullable:
                    case ConversionKind.ExplicitReference:
                    case ConversionKind.Identity:
                    case ConversionKind.ImplicitReference:
                    case ConversionKind.Unboxing:
                    case ConversionKind.NullLiteral:
                    case ConversionKind.ImplicitNullable:
                        // these are the conversions allowed by a pattern match
                        break;
                    //case ConversionKind.ExplicitNumeric:  // we do not perform numeric conversions of the operand
                    //case ConversionKind.ImplicitConstant:
                    //case ConversionKind.ImplicitNumeric:
                    default:
                        Error(diagnostics, ErrorCode.ERR_PatternWrongType, typeSyntax, operandType, patternType);
                        return true;
                }
            }

            return false;
        }

        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 bool LowerBoundNullableInference(TypeSymbol source, TypeSymbol target, ref HashSet<DiagnosticInfo> useSiteDiagnostics)
        {
            Debug.Assert((object)source != null);
            Debug.Assert((object)target != null);

            if (!source.IsNullableType() || !target.IsNullableType())
            {
                return false;
            }

            LowerBoundInference(source.GetNullableUnderlyingType(), target.GetNullableUnderlyingType(), ref useSiteDiagnostics);
            return true;
        }

        private bool LowerBoundNullableInference(TypeSymbol source, TypeSymbol target, ref HashSet<DiagnosticInfo> useSiteDiagnostics)
        {
            Debug.Assert((object)source != null);
            Debug.Assert((object)target != null);

            // SPEC ISSUE: As noted above, the spec does not clearly call out how
            // SPEC ISSUE: to do type inference to a nullable target. I propose the
            // SPEC ISSUE: following:
            // SPEC ISSUE:
            // SPEC ISSUE: * Otherwise, if V is nullable type V1? and U is a 
            // SPEC ISSUE:   non-nullable struct type then an exact inference is made from U to V1.

            if (!target.IsNullableType() || !source.IsValueType || source.IsNullableType())
            {
                return false;
            }

            ExactInference(source, target.GetNullableUnderlyingType(), ref useSiteDiagnostics);
            return true;
        }

        private BoundExpression CreateTupleLiteralConversion(CSharpSyntaxNode syntax, BoundTupleLiteral sourceTuple, Conversion conversion, bool isCast, TypeSymbol destination, DiagnosticBag diagnostics)
        {
            // We have a successful tuple conversion; rather than producing a separate conversion node 
            // which is a conversion on top of a tuple literal, tuple conversion is an element-wise conversion of arguments.

            Debug.Assert(conversion.Kind == ConversionKind.ImplicitTupleLiteral || conversion.Kind == ConversionKind.ImplicitNullable);
            Debug.Assert((conversion.Kind == ConversionKind.ImplicitNullable) == destination.IsNullableType());

            TypeSymbol destinationWithoutNullable = conversion.Kind == ConversionKind.ImplicitNullable ?
                                                                           destinationWithoutNullable = destination.GetNullableUnderlyingType() :
                                                                           destination;

            NamedTypeSymbol targetType = (NamedTypeSymbol)destinationWithoutNullable;

            if (targetType.IsTupleType)
            {
                var destTupleType = (TupleTypeSymbol)targetType;
                // do not lose the original element names in the literal if different from names in the target

                // Come back to this, what about locations? (https://github.com/dotnet/roslyn/issues/11013)
                targetType = destTupleType.WithElementNames(sourceTuple.ArgumentNamesOpt);
            }

            var arguments = sourceTuple.Arguments;
            var convertedArguments = ArrayBuilder<BoundExpression>.GetInstance(arguments.Length);

            ImmutableArray<TypeSymbol> targetElementTypes = targetType.GetElementTypesOfTupleOrCompatible();
            Debug.Assert(targetElementTypes.Length == arguments.Length, "converting a tuple literal to incompatible type?");

            for (int i = 0; i < arguments.Length; i++)
            {
                var argument = arguments[i];
                var destType = targetElementTypes[i];

                HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                Conversion elementConversion;
                if (isCast)
                {
                    elementConversion = this.Conversions.ClassifyConversionForCast(argument, destType, ref useSiteDiagnostics);
                }
                else
                {
                    elementConversion = this.Conversions.ClassifyConversionFromExpression(argument, destType, ref useSiteDiagnostics);
                }

                diagnostics.Add(syntax, useSiteDiagnostics);
                convertedArguments.Add(CreateConversion(argument.Syntax, argument, elementConversion, isCast, destType, diagnostics));
            }

            BoundExpression result = new BoundConvertedTupleLiteral(
                sourceTuple.Syntax,
                sourceTuple.Type,
                convertedArguments.ToImmutableAndFree(), 
                targetType);

            // We need to preserve any conversion that changes the type (even identity conversions),
            // or that was explicitly written in code (so that GetSemanticInfo can find the syntax in the bound tree).
            if (!isCast && targetType == destination)
            {
                return result;
            }

            // if we have a nullable cast combined with a name/dynamic cast
            // name/dynamic cast must happen before converting to nullable
            if (conversion.Kind == ConversionKind.ImplicitNullable &&
                destinationWithoutNullable != targetType)
            {
                Debug.Assert(destinationWithoutNullable.Equals(targetType, ignoreDynamic: true));

                result = new BoundConversion(
                    syntax,
                    result,
                    Conversion.Identity,
                    @checked: false,
                    explicitCastInCode: isCast,
                    constantValueOpt: ConstantValue.NotAvailable,
                    type: destinationWithoutNullable)
                { WasCompilerGenerated = sourceTuple.WasCompilerGenerated };
            }
                                  
            return new BoundConversion(
                syntax,
                result,
                conversion,
                @checked: false,
                explicitCastInCode: isCast,
                constantValueOpt: ConstantValue.NotAvailable,
                type: destination)
            { WasCompilerGenerated = sourceTuple.WasCompilerGenerated };
        }

        private BoundExpression CreateTupleLiteralConversion(SyntaxNode syntax, BoundTupleLiteral sourceTuple, Conversion conversion, bool isCast, TypeSymbol destination, DiagnosticBag diagnostics)
        {
            // We have a successful tuple conversion; rather than producing a separate conversion node 
            // which is a conversion on top of a tuple literal, tuple conversion is an element-wise conversion of arguments.
            Debug.Assert((conversion.Kind == ConversionKind.ImplicitNullable) == destination.IsNullableType());

            var destinationWithoutNullable = destination;
            var conversionWithoutNullable = conversion;

            if (conversion.Kind == ConversionKind.ImplicitNullable)
            {
                destinationWithoutNullable = destination.GetNullableUnderlyingType();
                conversionWithoutNullable = conversion.UnderlyingConversions[0];
            }

            Debug.Assert(conversionWithoutNullable.IsTupleLiteralConversion);

            NamedTypeSymbol targetType = (NamedTypeSymbol)destinationWithoutNullable;
            if (targetType.IsTupleType)
            {
                var destTupleType = (TupleTypeSymbol)targetType;
                // do not lose the original element names in the literal if different from names in the target

                TupleTypeSymbol.ReportNamesMismatchesIfAny(targetType, sourceTuple, diagnostics);

                // Come back to this, what about locations? (https://github.com/dotnet/roslyn/issues/11013)
                targetType = destTupleType.WithElementNames(sourceTuple.ArgumentNamesOpt);
            }

            var arguments = sourceTuple.Arguments;
            var convertedArguments = ArrayBuilder<BoundExpression>.GetInstance(arguments.Length);

            ImmutableArray<TypeSymbol> targetElementTypes = targetType.GetElementTypesOfTupleOrCompatible();
            Debug.Assert(targetElementTypes.Length == arguments.Length, "converting a tuple literal to incompatible type?");
            var underlyingConversions = conversionWithoutNullable.UnderlyingConversions;

            for (int i = 0; i < arguments.Length; i++)
            {
                var argument = arguments[i];
                var destType = targetElementTypes[i];
                var elementConversion = underlyingConversions[i];

                convertedArguments.Add(CreateConversion(argument.Syntax, argument, elementConversion, isCast, destType, diagnostics));
            }

            BoundExpression result = new BoundConvertedTupleLiteral(
                sourceTuple.Syntax,
                sourceTuple.Type,
                convertedArguments.ToImmutableAndFree(),
                targetType);

            if (sourceTuple.Type != destination)
            {
                // literal cast is applied to the literal 
                result = new BoundConversion(
                    sourceTuple.Syntax,
                    result,
                    conversion,
                    @checked: false,
                    explicitCastInCode: isCast,
                    constantValueOpt: ConstantValue.NotAvailable,
                    type: destination);
            }

            // If we had a cast in the code, keep conversion in the tree.
            // even though the literal is already converted to the target type.
            if (isCast)
            {
                result = new BoundConversion(
                    syntax,
                    result,
                    Conversion.Identity,
                    @checked: false,
                    explicitCastInCode: isCast,
                    constantValueOpt: ConstantValue.NotAvailable,
                    type: destination);
            }

            return result;
        }

        private static bool SatisfiesConstraintType(
            ConversionsBase conversions,
            TypeSymbol typeArgument,
            TypeSymbol constraintType,
            ref HashSet<DiagnosticInfo> useSiteDiagnostics)
        {
            if (constraintType.IsErrorType())
            {
                return false;
            }

            // Spec 4.4.4 describes the valid conversions from
            // type argument A to constraint type C:

            // "An identity conversion (6.1.1).
            // An implicit reference conversion (6.1.6). ..."
            if (conversions.HasIdentityOrImplicitReferenceConversion(typeArgument, constraintType, ref useSiteDiagnostics))
            {
                return true;
            }

            // "... A boxing conversion (6.1.7), provided that type A is a non-nullable value type. ..."
            // NOTE: we extend this to allow, for example, a conversion from Nullable<T> to object.
            if (typeArgument.IsValueType &&
                conversions.HasBoxingConversion(typeArgument.IsNullableType() ? ((NamedTypeSymbol)typeArgument).ConstructedFrom : typeArgument, constraintType, ref useSiteDiagnostics))
            {
                return true;
            }

            if (typeArgument.TypeKind == TypeKind.TypeParameter)
            {
                var typeParameter = (TypeParameterSymbol)typeArgument;

                // "... An implicit reference, boxing, or type parameter conversion
                // from type parameter A to C."
                if (conversions.HasImplicitTypeParameterConversion(typeParameter, constraintType, ref useSiteDiagnostics))
                {
                    return true;
                }

                // TypeBind::SatisfiesBound allows cases where one of the
                // type parameter constraints satisfies the constraint.
                foreach (var typeArgumentConstraint in typeParameter.ConstraintTypesWithDefinitionUseSiteDiagnostics(ref useSiteDiagnostics))
                {
                    if (SatisfiesConstraintType(conversions, typeArgumentConstraint, constraintType, ref useSiteDiagnostics))
                    {
                        return true;
                    }
                }
            }

            return false;
        }