private void AddUserDefinedConversionsToExplicitCandidateSet( BoundExpression sourceExpression, TypeSymbol source, TypeSymbol target, ArrayBuilder <UserDefinedConversionAnalysis> u, TypeParameterSymbol constrainedToTypeOpt, NamedTypeSymbol declaringType, bool isExplicit, bool isChecked, ref CompoundUseSiteInfo <AssemblySymbol> useSiteInfo) { Debug.Assert(sourceExpression is null || Compilation is not null); Debug.Assert(sourceExpression != null || (object)source != null); Debug.Assert((object)target != null); Debug.Assert(u != null); Debug.Assert((object)declaringType != 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; } ImmutableArray <MethodSymbol> operators = declaringType.GetOperators( isExplicit ? (isChecked ? WellKnownMemberNames.CheckedExplicitConversionName : WellKnownMemberNames.ExplicitConversionName) : WellKnownMemberNames.ImplicitConversionName); var candidates = ArrayBuilder <MethodSymbol> .GetInstance(operators.Length); candidates.AddRange(operators); if (isExplicit && isChecked) { ImmutableArray <MethodSymbol> operators2 = declaringType.GetOperators(WellKnownMemberNames.ExplicitConversionName); // Add regular operators as well. if (operators.IsEmpty) { candidates.AddRange(operators2); } else { foreach (MethodSymbol op2 in operators2) { // Drop operators that have a match among the checked ones. bool add = true; foreach (MethodSymbol op in operators) { if (SourceMemberContainerTypeSymbol.DoOperatorsPair(op, op2)) { add = false; break; } } if (add) { candidates.Add(op2); } } } } foreach (MethodSymbol op in candidates) { // 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.GetParameterType(0); TypeSymbol convertsTo = op.ReturnType; Conversion fromConversion = EncompassingExplicitConversion(sourceExpression, source, convertsFrom, ref useSiteInfo); Conversion toConversion = EncompassingExplicitConversion(convertsTo, target, ref useSiteInfo); // We accept candidates for which the parameter type encompasses the *underlying* source type. if (!fromConversion.Exists && (object)source != null && source.IsNullableType() && EncompassingExplicitConversion(source.GetNullableUnderlyingType(), convertsFrom, ref useSiteInfo).Exists) { fromConversion = ClassifyBuiltInConversion(source, convertsFrom, isChecked: isChecked, ref useSiteInfo); } // 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(convertsTo, target.GetNullableUnderlyingType(), ref useSiteInfo).Exists) { toConversion = ClassifyBuiltInConversion(convertsTo, target, isChecked: isChecked, ref useSiteInfo); } // In the corresponding implicit conversion code we can get away with first // checking to see if standard implicit conversions exist from the source type // to the parameter type, and from the return type to the target type. If not, // then we can check for a lifted operator. // // That's not going to cut it in the explicit conversion code. Suppose we have // a conversion X-->Y and have source type X? and target type Y?. There *are* // standard explicit conversions from X?-->X and Y?-->Y, but we do not want // to bind this as an *unlifted* conversion from X? to Y?; we want such a thing // to be a *lifted* conversion from X? to Y?, that checks for null on the source // and decides to not call the underlying user-defined conversion if it is null. // // We therefore cannot do what we do in the implicit conversions, where we check // to see if the unlifted conversion works, and if it does, then don't add the lifted // conversion at all. Rather, we have to see if what we're building here is a // lifted conversion or not. // // Under what circumstances is this conversion a lifted conversion? (In the // "spec" sense of a lifted conversion; that is, that we check for null // and skip the user-defined conversion if necessary). // // * The source type must be a nullable value type. // * The parameter type must be a non-nullable value type. // * The target type must be able to take on a null value. if (fromConversion.Exists && toConversion.Exists) { if ((object)source != null && source.IsNullableType() && convertsFrom.IsValidNullableTypeArgument() && target.CanBeAssignedNull()) { TypeSymbol nullableFrom = MakeNullableType(convertsFrom); TypeSymbol nullableTo = convertsTo.IsValidNullableTypeArgument() ? MakeNullableType(convertsTo) : convertsTo; Conversion liftedFromConversion = EncompassingExplicitConversion(sourceExpression, source, nullableFrom, ref useSiteInfo); Conversion liftedToConversion = EncompassingExplicitConversion(nullableTo, target, ref useSiteInfo); Debug.Assert(liftedFromConversion.Exists); Debug.Assert(liftedToConversion.Exists); u.Add(UserDefinedConversionAnalysis.Lifted(constrainedToTypeOpt, op, liftedFromConversion, liftedToConversion, nullableFrom, nullableTo)); } else { // There is an additional spec violation in the native compiler. Suppose // we have a conversion from X-->Y and are asked to do "Y? y = new X();" Clearly // the intention is to convert from X-->Y via the implicit conversion, and then // stick a standard implicit conversion from Y-->Y? on the back end. **In this // situation, the native compiler treats the conversion as though it were // actually X-->Y? in source for the purposes of determining the best target // type of a set of operators. // // Similarly, if we have a conversion from X-->Y and are asked to do // an explicit conversion from X? to Y then we treat the conversion as // though it really were X?-->Y for the purposes of determining the best // source type of a set of operators. // // We perpetuate these fictions here, except when X or Y is not a valid // type argument to `Nullable<T>`. if (target.IsNullableType() && convertsTo.IsValidNullableTypeArgument()) { convertsTo = MakeNullableType(convertsTo); toConversion = EncompassingExplicitConversion(convertsTo, target, ref useSiteInfo); } if ((object)source != null && source.IsNullableType() && convertsFrom.IsValidNullableTypeArgument()) { convertsFrom = MakeNullableType(convertsFrom); fromConversion = EncompassingExplicitConversion(convertsFrom, source, ref useSiteInfo); } u.Add(UserDefinedConversionAnalysis.Normal(constrainedToTypeOpt, op, fromConversion, toConversion, convertsFrom, convertsTo)); } } } candidates.Free(); }