//
// ecore.cs: Core of the Expression representation for the intermediate tree.
//
// Author:
//   Miguel de Icaza (miguel@ximian.com)
//
// (C) 2001 Ximian, Inc.
//
//

namespace CIR {
	using System;
	using System.Collections;
	using System.Diagnostics;
	using System.Reflection;
	using System.Reflection.Emit;
	using System.Text;

	// <remarks>
	//   The ExprClass class contains the is used to pass the 
	//   classification of an expression (value, variable, namespace,
	//   type, method group, property access, event access, indexer access,
	//   nothing).
	// </remarks>
	public enum ExprClass {
		Invalid,
		
		Value,
		Variable,
		Namespace,
		Type,
		MethodGroup,
		PropertyAccess,
		EventAccess,
		IndexerAccess,
		Nothing, 
	}

	// <summary>
	//   An interface provided by expressions that can be used as
	//   LValues and can store the value on the top of the stack on
	//   their storage
	// </summary>
	public interface IStackStore {

		// <summary>
		//   The Store method should store the contents of the top
		//   of the stack into the storage that is implemented by
		//   the particular implementation of LValue
		// </summary>
		void Store     (EmitContext ec);
	}

	// <summary>
	//   This interface is implemented by variables
	// </summary>
	public interface IMemoryLocation {
		// <summary>
		//   The AddressOf method should generate code that loads
		//   the address of the object and leaves it on the stack
		// </summary>
		void AddressOf (EmitContext ec);
	}

	// <remarks>
	//   Base class for expressions
	// </remarks>
	public abstract class Expression {
		protected ExprClass eclass;
		protected Type      type;
		
		public Type Type {
			get {
				return type;
			}

			set {
				type = value;
			}
		}

		public ExprClass ExprClass {
			get {
				return eclass;
			}

			set {
				eclass = value;
			}
		}

		// <summary>
		//   Utility wrapper routine for Error, just to beautify the code
		// </summary>
		static protected void Error (int error, string s)
		{
			Report.Error (error, s);
		}

		static protected void Error (int error, Location loc, string s)
		{
			Report.Error (error, loc, s);
		}
		
		// <summary>
		//   Utility wrapper routine for Warning, just to beautify the code
		// </summary>
		static protected void Warning (int warning, string s)
		{
			Report.Warning (warning, s);
		}

		// <summary>
		//   Performs semantic analysis on the Expression
		// </summary>
		//
		// <remarks>
		//   The Resolve method is invoked to perform the semantic analysis
		//   on the node.
		//
		//   The return value is an expression (it can be the
		//   same expression in some cases) or a new
		//   expression that better represents this node.
		//   
		//   For example, optimizations of Unary (LiteralInt)
		//   would return a new LiteralInt with a negated
		//   value.
		//   
		//   If there is an error during semantic analysis,
		//   then an error should be reported (using Report)
		//   and a null value should be returned.
		//   
		//   There are two side effects expected from calling
		//   Resolve(): the the field variable "eclass" should
		//   be set to any value of the enumeration
		//   `ExprClass' and the type variable should be set
		//   to a valid type (this is the type of the
		//   expression).
		// </remarks>
		
		public abstract Expression DoResolve (EmitContext ec);

		public virtual Expression DoResolveLValue (EmitContext ec, Expression right_side)
		{
			return DoResolve (ec);
		}
		
		//
		// Currently Resolve wraps DoResolve to perform sanity
		// checking and assertion checking on what we expect from Resolve
		//
		public Expression Resolve (EmitContext ec)
		{
			Expression e = DoResolve (ec);

			if (e != null){
				if (e is SimpleName){
					SimpleName s = (SimpleName) e;
					
					Report.Error (
						103, s.Location,
						"The name `" + s.Name + "' could not be found in `" +
						ec.TypeContainer.Name + "'");
					return null;
				}
				
				if (e.ExprClass == ExprClass.Invalid)
					throw new Exception ("Expression " + e +
							     " ExprClass is Invalid after resolve");

				if (e.ExprClass != ExprClass.MethodGroup)
					if (e.type == null)
						throw new Exception ("Expression " + e +
								     " did not set its type after Resolve");
			}

			return e;
		}

		//
		// Just like `Resolve' above, but this allows SimpleNames to be returned.
		// This is used by MemberAccess to construct long names that can not be
		// partially resolved (namespace-qualified names for example).
		//
		public Expression ResolveWithSimpleName (EmitContext ec)
		{
			Expression e = DoResolve (ec);

			if (e != null){
				if (e is SimpleName)
					return e;

				if (e.ExprClass == ExprClass.Invalid)
					throw new Exception ("Expression " + e +
							     " ExprClass is Invalid after resolve");

				if (e.ExprClass != ExprClass.MethodGroup)
					if (e.type == null)
						throw new Exception ("Expression " + e +
								     " did not set its type after Resolve");
			}

			return e;
		}
		
		//
		// Currently ResolveLValue wraps DoResolveLValue to perform sanity
		// checking and assertion checking on what we expect from Resolve
		//
		public Expression ResolveLValue (EmitContext ec, Expression right_side)
		{
			Expression e = DoResolveLValue (ec, right_side);

			if (e != null){
				if (e is SimpleName){
					SimpleName s = (SimpleName) e;
					
					Report.Error (
						103, s.Location,
						"The name `" + s.Name + "' could not be found in `" +
						ec.TypeContainer.Name + "'");
					return null;
				}

				if (e.ExprClass == ExprClass.Invalid)
					throw new Exception ("Expression " + e +
							     " ExprClass is Invalid after resolve");

				if (e.ExprClass != ExprClass.MethodGroup)
					if (e.type == null)
						throw new Exception ("Expression " + e +
								     " did not set its type after Resolve");
			}

			return e;
		}
		
		// <summary>
		//   Emits the code for the expression
		// </summary>
		//
		// <remarks>
		// 
		//   The Emit method is invoked to generate the code
		//   for the expression.  
		//
		// </remarks>
		public abstract void Emit (EmitContext ec);

		// <summary>
		//   This method should perform a reduction of the expression.  This should
		//   never return null.
		// </summary>
		public virtual Expression Reduce (EmitContext ec)
		{
			return this;
		}

		// <summary>
		//   Protected constructor.  Only derivate types should
		//   be able to be created
		// </summary>

		protected Expression ()
		{
			eclass = ExprClass.Invalid;
			type = null;
		}

		// <summary>
		//   Returns a literalized version of a literal FieldInfo
		// </summary>
		public static Expression Literalize (object v, Type t)
		{
			if (t == TypeManager.int32_type)
				return new IntLiteral ((int) v);
			else if (t == TypeManager.uint32_type)
				return new UIntLiteral ((uint) v);
			else if (t == TypeManager.int64_type)
				return new LongLiteral ((long) v);
			else if (t == TypeManager.uint64_type)
				return new ULongLiteral ((ulong) v);
			else if (t == TypeManager.float_type)
				return new FloatLiteral ((float) v);
			else if (t == TypeManager.double_type)
				return new DoubleLiteral ((double) v);
			else if (t == TypeManager.string_type)
				return new StringLiteral ((string) v);
			else if (t == TypeManager.short_type)
				return new IntLiteral ((int) ((short)v));
			else if (t == TypeManager.ushort_type)
				return new IntLiteral ((int) ((ushort)v));
			else if (t == TypeManager.sbyte_type)
				return new IntLiteral ((int) ((sbyte)v));
			else if (t == TypeManager.byte_type)
				return new IntLiteral ((int) ((byte)v));
			else if (t == TypeManager.char_type)
				return new IntLiteral ((int) ((char)v));
			else
				throw new Exception ("Unknown type for literal (" + t +
						     "), details: " + v);
		}

		// 
		// Returns a fully formed expression after a MemberLookup
		//
		static Expression ExprClassFromMemberInfo (EmitContext ec, MemberInfo mi, Location loc)
		{
			if (mi is EventInfo)
				return new EventExpr ((EventInfo) mi, loc);
			else if (mi is FieldInfo)
				return new FieldExpr ((FieldInfo) mi, loc);
			else if (mi is PropertyInfo)
				return new PropertyExpr ((PropertyInfo) mi, loc);
		        else if (mi is Type)
				return new TypeExpr ((Type) mi);

			return null;
		}

		//
		// FIXME: Probably implement a cache for (t,name,current_access_set)?
		//
		// FIXME: We need to cope with access permissions here, or this wont
		// work!
		//
		// This code could use some optimizations, but we need to do some
		// measurements.  For example, we could use a delegate to `flag' when
		// something can not any longer be a method-group (because it is something
		// else).
		//
		// Return values:
		//     If the return value is an Array, then it is an array of
		//     MethodBases
		//   
		//     If the return value is an MemberInfo, it is anything, but a Method
		//
		//     null on error.
		//
		// FIXME: When calling MemberLookup inside an `Invocation', we should pass
		// the arguments here and have MemberLookup return only the methods that
		// match the argument count/type, unlike we are doing now (we delay this
		// decision).
		//
		// This is so we can catch correctly attempts to invoke instance methods
		// from a static body (scan for error 120 in ResolveSimpleName).
		//
		public static Expression MemberLookup (EmitContext ec, Type t, string name,
						       bool same_type, MemberTypes mt,
						       BindingFlags bf, Location loc)
		{
			if (same_type)
				bf |= BindingFlags.NonPublic;

			MemberInfo [] mi = ec.TypeContainer.RootContext.TypeManager.FindMembers (
				t, mt, bf, Type.FilterName, name);

			if (mi == null)
				return null;

			// Empty array ...
			if (mi.Length == 0) 
				return null;

			
			if (mi.Length == 1 && !(mi [0] is MethodBase))
				return Expression.ExprClassFromMemberInfo (ec, mi [0], loc);
			
			for (int i = 0; i < mi.Length; i++)
				if (!(mi [i] is MethodBase)){
					Error (-5, "Do not know how to reproduce this case: " + 
					       "Methods and non-Method with the same name, " +
					       "report this please");

					for (i = 0; i < mi.Length; i++){
						Type tt = mi [i].GetType ();

						Console.WriteLine (i + ": " + mi [i]);
						while (tt != TypeManager.object_type){
							Console.WriteLine (tt);
							tt = tt.BaseType;
						}
					}
				}

			return new MethodGroupExpr (mi);
		}

		public const MemberTypes AllMemberTypes =
			MemberTypes.Constructor |
			MemberTypes.Event       |
			MemberTypes.Field       |
			MemberTypes.Method      |
			MemberTypes.NestedType  |
			MemberTypes.Property;
		
		public const BindingFlags AllBindingsFlags =
			BindingFlags.Public |
			BindingFlags.Static |
			BindingFlags.Instance;

		public static Expression MemberLookup (EmitContext ec, Type t, string name,
						       bool same_type, Location loc)
		{
			return MemberLookup (ec, t, name, same_type, AllMemberTypes, AllBindingsFlags, loc);
		}

		static public Expression ImplicitReferenceConversion (Expression expr, Type target_type)
		{
			Type expr_type = expr.Type;

			if (target_type == TypeManager.object_type) {
				if (expr_type.IsClass)
					return new EmptyCast (expr, target_type);
				if (expr_type.IsValueType)
					return new BoxedCast (expr);
			} else if (expr_type.IsSubclassOf (target_type)) {
				return new EmptyCast (expr, target_type);
			} else {
				// from any class-type S to any interface-type T.
				if (expr_type.IsClass && target_type.IsInterface) {

					if (TypeManager.ImplementsInterface (expr_type, target_type))
						return new EmptyCast (expr, target_type);
					else
						return null;
				}

				// from any interface type S to interface-type T.
				if (expr_type.IsInterface && target_type.IsInterface) {

					if (TypeManager.ImplementsInterface (expr_type, target_type))
						return new EmptyCast (expr, target_type);
					else
						return null;
				}
				
				// from an array-type S to an array-type of type T
				if (expr_type.IsArray && target_type.IsArray) {
					if (expr_type.GetArrayRank () == target_type.GetArrayRank ()) {

						Type expr_element_type = expr_type.GetElementType ();
						Type target_element_type = target_type.GetElementType ();

						if (!expr_element_type.IsValueType && !target_element_type.IsValueType)
							if (StandardConversionExists (expr_element_type,
										      target_element_type))
								return new EmptyCast (expr, target_type);
					}
				}
				
				
				// from an array-type to System.Array
				if (expr_type.IsArray && target_type == TypeManager.array_type)
					return new EmptyCast (expr, target_type);
				
				// from any delegate type to System.Delegate
				if (expr_type.IsSubclassOf (TypeManager.delegate_type) &&
				    target_type == TypeManager.delegate_type)
					return new EmptyCast (expr, target_type);
					
				// from any array-type or delegate type into System.ICloneable.
				if (expr_type.IsArray || expr_type.IsSubclassOf (TypeManager.delegate_type))
					if (target_type == TypeManager.icloneable_type)
						return new EmptyCast (expr, target_type);
				
				// from the null type to any reference-type.
				if (expr is NullLiteral)
					return new EmptyCast (expr, target_type);

				return null;

			}
			
			return null;
		}

		// <summary>
		//   Handles expressions like this: decimal d; d = 1;
		//   and changes them into: decimal d; d = new System.Decimal (1);
		// </summary>
		static Expression InternalTypeConstructor (EmitContext ec, Expression expr, Type target)
		{
			ArrayList args = new ArrayList ();

			args.Add (new Argument (expr, Argument.AType.Expression));

			Expression ne = new New (target.FullName, args,
						 new Location (-1));

			return ne.Resolve (ec);
		}

		// <summary>
		//   Implicit Numeric Conversions.
		//
		//   expr is the expression to convert, returns a new expression of type
		//   target_type or null if an implicit conversion is not possible.
		//
		// </summary>
		static public Expression ImplicitNumericConversion (EmitContext ec, Expression expr,
								    Type target_type, Location loc)
		{
			Type expr_type = expr.Type;
			
			//
			// Attempt to do the implicit constant expression conversions

			if (expr is IntLiteral){
				Expression e;
				
				e = TryImplicitIntConversion (target_type, (IntLiteral) expr);
				if (e != null)
					return e;
			} else if (expr is LongLiteral){
				//
				// Try the implicit constant expression conversion
				// from long to ulong, instead of a nice routine,
				// we just inline it
				//
				if (((LongLiteral) expr).Value > 0)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
			}
			
			if (expr_type == TypeManager.sbyte_type){
				//
				// From sbyte to short, int, long, float, double.
				//
				if (target_type == TypeManager.int32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I4);
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R8);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R4);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.byte_type){
				//
				// From byte to short, ushort, int, uint, long, ulong, float, double
				// 
				if ((target_type == TypeManager.short_type) ||
				    (target_type == TypeManager.ushort_type) ||
				    (target_type == TypeManager.int32_type) ||
				    (target_type == TypeManager.uint32_type))
					return new EmptyCast (expr, target_type);

				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U8);
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R4);
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R8);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.short_type){
				//
				// From short to int, long, float, double
				// 
				if (target_type == TypeManager.int32_type)
					return new EmptyCast (expr, target_type);
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R8);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R4);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.ushort_type){
				//
				// From ushort to int, uint, long, ulong, float, double
				//
				if (target_type == TypeManager.uint32_type)
					return new EmptyCast (expr, target_type);

				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U8);
				if (target_type == TypeManager.int32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I4);
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R8);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R4);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.int32_type){
				//
				// From int to long, float, double
				//
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R8);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R4);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.uint32_type){
				//
				// From uint to long, ulong, float, double
				//
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U8);
				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U8);
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R_Un,
							       OpCodes.Conv_R8);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R_Un,
							       OpCodes.Conv_R4);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if ((expr_type == TypeManager.uint64_type) ||
				   (expr_type == TypeManager.int64_type)){
				//
				// From long/ulong to float, double
				//
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R_Un,
							       OpCodes.Conv_R8);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R_Un,
							       OpCodes.Conv_R4);	
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.char_type){
				//
				// From char to ushort, int, uint, long, ulong, float, double
				// 
				if ((target_type == TypeManager.ushort_type) ||
				    (target_type == TypeManager.int32_type) ||
				    (target_type == TypeManager.uint32_type))
					return new EmptyCast (expr, target_type);
				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U8);
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R4);
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R8);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.float_type){
				//
				// float to double
				//
				if (target_type == TypeManager.double_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R8);
			}

			return null;
		}

		// <summary>
		//  Determines if a standard implicit conversion exists from
		//  expr_type to target_type
		// </summary>
		public static bool StandardConversionExists (Type expr_type, Type target_type)
		{
			if (expr_type == target_type)
				return true;

			// First numeric conversions 
			
			if (expr_type == TypeManager.sbyte_type){
				//
				// From sbyte to short, int, long, float, double.
				//
				if ((target_type == TypeManager.int32_type) || 
				    (target_type == TypeManager.int64_type) ||
				    (target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.float_type)  ||
				    (target_type == TypeManager.short_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;
				
			} else if (expr_type == TypeManager.byte_type){
				//
				// From byte to short, ushort, int, uint, long, ulong, float, double
				// 
				if ((target_type == TypeManager.short_type) ||
				    (target_type == TypeManager.ushort_type) ||
				    (target_type == TypeManager.int32_type) ||
				    (target_type == TypeManager.uint32_type) ||
				    (target_type == TypeManager.uint64_type) ||
				    (target_type == TypeManager.int64_type) ||
				    (target_type == TypeManager.float_type) ||
				    (target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;
	
			} else if (expr_type == TypeManager.short_type){
				//
				// From short to int, long, float, double
				// 
				if ((target_type == TypeManager.int32_type) ||
				    (target_type == TypeManager.int64_type) ||
				    (target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.float_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;
					
			} else if (expr_type == TypeManager.ushort_type){
				//
				// From ushort to int, uint, long, ulong, float, double
				//
				if ((target_type == TypeManager.uint32_type) ||
				    (target_type == TypeManager.uint64_type) ||
				    (target_type == TypeManager.int32_type) ||
				    (target_type == TypeManager.int64_type) ||
				    (target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.float_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;
				    
			} else if (expr_type == TypeManager.int32_type){
				//
				// From int to long, float, double
				//
				if ((target_type == TypeManager.int64_type) ||
				    (target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.float_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;
					
			} else if (expr_type == TypeManager.uint32_type){
				//
				// From uint to long, ulong, float, double
				//
				if ((target_type == TypeManager.int64_type) ||
				    (target_type == TypeManager.uint64_type) ||
				    (target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.float_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;
					
			} else if ((expr_type == TypeManager.uint64_type) ||
				   (expr_type == TypeManager.int64_type)) {
				//
				// From long/ulong to float, double
				//
				if ((target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.float_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;
				    
			} else if (expr_type == TypeManager.char_type){
				//
				// From char to ushort, int, uint, long, ulong, float, double
				// 
				if ((target_type == TypeManager.ushort_type) ||
				    (target_type == TypeManager.int32_type) ||
				    (target_type == TypeManager.uint32_type) ||
				    (target_type == TypeManager.uint64_type) ||
				    (target_type == TypeManager.int64_type) ||
				    (target_type == TypeManager.float_type) ||
				    (target_type == TypeManager.double_type) ||
				    (target_type == TypeManager.decimal_type))
					return true;

			} else if (expr_type == TypeManager.float_type){
				//
				// float to double
				//
				if (target_type == TypeManager.double_type)
					return true;
			}	
			
			// Next reference conversions

			if (target_type == TypeManager.object_type) {
				if ((expr_type.IsClass) ||
				    (expr_type.IsValueType))
					return true;
				
			} else if (expr_type.IsSubclassOf (target_type)) {
				return true;
				
			} else {
				// from any class-type S to any interface-type T.
				if (expr_type.IsClass && target_type.IsInterface)
					return true;
				
				// from any interface type S to interface-type T.
				// FIXME : Is it right to use IsAssignableFrom ?
				if (expr_type.IsInterface && target_type.IsInterface)
					if (target_type.IsAssignableFrom (expr_type))
						return true;
				
				// from an array-type S to an array-type of type T
				if (expr_type.IsArray && target_type.IsArray) {
					if (expr_type.GetArrayRank () == target_type.GetArrayRank ()) {
						
						Type expr_element_type = expr_type.GetElementType ();
						Type target_element_type = target_type.GetElementType ();
						
						if (!expr_element_type.IsValueType && !target_element_type.IsValueType)
							if (StandardConversionExists (expr_element_type,
										      target_element_type))
								return true;
					}
				}
				
				// from an array-type to System.Array
				if (expr_type.IsArray && target_type.IsAssignableFrom (expr_type))
					return true;
				
				// from any delegate type to System.Delegate
				if (expr_type.IsSubclassOf (TypeManager.delegate_type) &&
				    target_type == TypeManager.delegate_type)
					if (target_type.IsAssignableFrom (expr_type))
						return true;
					
				// from any array-type or delegate type into System.ICloneable.
				if (expr_type.IsArray || expr_type.IsSubclassOf (TypeManager.delegate_type))
					if (target_type == TypeManager.icloneable_type)
						return true;
				
				// from the null type to any reference-type.
				// FIXME : How do we do this ?

			}

			return false;
		}
		
		// <summary>
		//  Finds "most encompassed type" according to the spec (13.4.2)
		//  amongst the methods in the MethodGroupExpr which convert from a
		//  type encompassing source_type
		// </summary>
		static Type FindMostEncompassedType (MethodGroupExpr me, Type source_type)
		{
			Type best = null;
			
			for (int i = me.Methods.Length; i > 0; ) {
				i--;

				MethodBase mb = me.Methods [i];
				ParameterData pd = Invocation.GetParameterData (mb);
				Type param_type = pd.ParameterType (0);

				if (StandardConversionExists (source_type, param_type)) {
					if (best == null)
						best = param_type;
					
					if (StandardConversionExists (param_type, best))
						best = param_type;
				}
			}

			return best;
		}
		
		// <summary>
		//  Finds "most encompassing type" according to the spec (13.4.2)
		//  amongst the methods in the MethodGroupExpr which convert to a
		//  type encompassed by target_type
		// </summary>
		static Type FindMostEncompassingType (MethodGroupExpr me, Type target)
		{
			Type best = null;
			
			for (int i = me.Methods.Length; i > 0; ) {
				i--;
				
				MethodInfo mi = (MethodInfo) me.Methods [i];
				Type ret_type = mi.ReturnType;
				
				if (StandardConversionExists (ret_type, target)) {
					if (best == null)
						best = ret_type;

					if (!StandardConversionExists (ret_type, best))
						best = ret_type;
				}
				
			}
			
			return best;

		}
		

		// <summary>
		//  User-defined Implicit conversions
		// </summary>
		static public Expression ImplicitUserConversion (EmitContext ec, Expression source,
								 Type target, Location loc)
		{
			return UserDefinedConversion (ec, source, target, loc, false);
		}

		// <summary>
		//  User-defined Explicit conversions
		// </summary>
		static public Expression ExplicitUserConversion (EmitContext ec, Expression source,
								 Type target, Location loc)
		{
			return UserDefinedConversion (ec, source, target, loc, true);
		}
		
		// <summary>
		//   User-defined conversions
		// </summary>
		static public Expression UserDefinedConversion (EmitContext ec, Expression source,
								Type target, Location loc,
								bool look_for_explicit)
		{
			Expression mg1 = null, mg2 = null, mg3 = null, mg4 = null;
			Expression mg5 = null, mg6 = null, mg7 = null, mg8 = null;
			Expression e;
			MethodBase method = null;
			Type source_type = source.Type;

			string op_name;
			
			// If we have a boolean type, we need to check for the True operator

			// FIXME : How does the False operator come into the picture ?
			// FIXME : This doesn't look complete and very correct !
			if (target == TypeManager.bool_type)
				op_name = "op_True";
			else
				op_name = "op_Implicit";
			
			mg1 = MemberLookup (ec, source_type, op_name, false, loc);

			if (source_type.BaseType != null)
				mg2 = MemberLookup (ec, source_type.BaseType, op_name, false, loc);
			
			mg3 = MemberLookup (ec, target, op_name, false, loc);

			if (target.BaseType != null)
				mg4 = MemberLookup (ec, target.BaseType, op_name, false, loc);

			MethodGroupExpr union1 = Invocation.MakeUnionSet (mg1, mg2);
			MethodGroupExpr union2 = Invocation.MakeUnionSet (mg3, mg4);

			MethodGroupExpr union3 = Invocation.MakeUnionSet (union1, union2);

			MethodGroupExpr union4 = null;

			if (look_for_explicit) {

				op_name = "op_Explicit";
				
				mg5 = MemberLookup (ec, source_type, op_name, false, loc);

				if (source_type.BaseType != null)
					mg6 = MemberLookup (ec, source_type.BaseType, op_name, false, loc);
				
				mg7 = MemberLookup (ec, target, op_name, false, loc);
				
				if (target.BaseType != null)
					mg8 = MemberLookup (ec, target.BaseType, op_name, false, loc);
				
				MethodGroupExpr union5 = Invocation.MakeUnionSet (mg5, mg6);
				MethodGroupExpr union6 = Invocation.MakeUnionSet (mg7, mg8);

				union4 = Invocation.MakeUnionSet (union5, union6);
			}
			
			MethodGroupExpr union = Invocation.MakeUnionSet (union3, union4);

			if (union != null) {

				Type most_specific_source, most_specific_target;

				most_specific_source = FindMostEncompassedType (union, source_type);
				if (most_specific_source == null)
					return null;

				most_specific_target = FindMostEncompassingType (union, target);
				if (most_specific_target == null) 
					return null;
				
				int count = 0;
				
				for (int i = union.Methods.Length; i > 0;) {
					i--;

					MethodBase mb = union.Methods [i];
					ParameterData pd = Invocation.GetParameterData (mb);
					MethodInfo mi = (MethodInfo) union.Methods [i];

					if (pd.ParameterType (0) == most_specific_source &&
					    mi.ReturnType == most_specific_target) {
						method = mb;
						count++;
					}
				}

				if (method == null || count > 1) {
					Report.Error (-11, loc, "Ambiguous user defined conversion");
					return null;
				}
				
				//
				// This will do the conversion to the best match that we
				// found.  Now we need to perform an implict standard conversion
				// if the best match was not the type that we were requested
				// by target.
				//
				if (look_for_explicit)
					source = ConvertExplicitStandard (ec, source, most_specific_source, loc);
				else
					source = ConvertImplicitStandard (ec, source,
									  most_specific_source, loc);

				if (source == null)
					return null;
				
				e =  new UserCast ((MethodInfo) method, source);
				
				if (e.Type != target){
					if (!look_for_explicit)
						e = ConvertImplicitStandard (ec, e, target, loc);
					else
						e = ConvertExplicitStandard (ec, e, target, loc);

					return e;
				} else
					return e;
			}
			
			return null;
		}
		
		// <summary>
		//   Converts implicitly the resolved expression `expr' into the
		//   `target_type'.  It returns a new expression that can be used
		//   in a context that expects a `target_type'. 
		// </summary>
		static public Expression ConvertImplicit (EmitContext ec, Expression expr,
							  Type target_type, Location loc)
		{
			Type expr_type = expr.Type;
			Expression e;

			if (expr_type == target_type)
				return expr;

			e = ImplicitNumericConversion (ec, expr, target_type, loc);
			if (e != null)
				return e;

			e = ImplicitReferenceConversion (expr, target_type);
			if (e != null)
				return e;

			e = ImplicitUserConversion (ec, expr, target_type, loc);
			if (e != null)
				return e;

			if (target_type.IsSubclassOf (TypeManager.enum_type) && expr is IntLiteral){
				IntLiteral i = (IntLiteral) expr;

				if (i.Value == 0)
					return new EmptyCast (expr, target_type);
			}

			return null;
		}

		
		// <summary>
		//   Attempts to apply the `Standard Implicit
		//   Conversion' rules to the expression `expr' into
		//   the `target_type'.  It returns a new expression
		//   that can be used in a context that expects a
		//   `target_type'.
		//
		//   This is different from `ConvertImplicit' in that the
		//   user defined implicit conversions are excluded. 
		// </summary>
		static public Expression ConvertImplicitStandard (EmitContext ec, Expression expr,
								  Type target_type, Location loc)
		{
			Type expr_type = expr.Type;
			Expression e;

			if (expr_type == target_type)
				return expr;

			e = ImplicitNumericConversion (ec, expr, target_type, loc);
			if (e != null)
				return e;

			e = ImplicitReferenceConversion (expr, target_type);
			if (e != null)
				return e;

			if (target_type.IsSubclassOf (TypeManager.enum_type) && expr is IntLiteral){
				IntLiteral i = (IntLiteral) expr;

				if (i.Value == 0)
					return new EmptyCast (expr, target_type);
			}
			return null;
		}
		// <summary>
		//   Attemps to perform an implict constant conversion of the IntLiteral
		//   into a different data type using casts (See Implicit Constant
		//   Expression Conversions)
		// </summary>
		static protected Expression TryImplicitIntConversion (Type target_type, IntLiteral il)
		{
			int value = il.Value;
			
			if (target_type == TypeManager.sbyte_type){
				if (value >= SByte.MinValue && value <= SByte.MaxValue)
					return il;
			} else if (target_type == TypeManager.byte_type){
				if (Byte.MinValue >= 0 && value <= Byte.MaxValue)
					return il;
			} else if (target_type == TypeManager.short_type){
				if (value >= Int16.MinValue && value <= Int16.MaxValue)
					return il;
			} else if (target_type == TypeManager.ushort_type){
				if (value >= UInt16.MinValue && value <= UInt16.MaxValue)
					return il;
			} else if (target_type == TypeManager.uint32_type){
				//
				// we can optimize this case: a positive int32
				// always fits on a uint32
				//
				if (value >= 0)
					return il;
			} else if (target_type == TypeManager.uint64_type){
				//
				// we can optimize this case: a positive int32
				// always fits on a uint64.  But we need an opcode
				// to do it.
				//
				if (value >= 0)
					return new OpcodeCast (il, target_type, OpCodes.Conv_I8);
			}

			return null;
		}

		// <summary>
		//   Attemptes to implicityly convert `target' into `type', using
		//   ConvertImplicit.  If there is no implicit conversion, then
		//   an error is signaled
		// </summary>
		static public Expression ConvertImplicitRequired (EmitContext ec, Expression target,
								  Type type, Location loc)
		{
			Expression e;
			
			e = ConvertImplicit (ec, target, type, loc);
			if (e != null)
				return e;
			
			string msg = "Can not convert implicitly from `"+
				TypeManager.CSharpName (target.Type) + "' to `" +
				TypeManager.CSharpName (type) + "'";

			Error (29, loc, msg);

			return null;
		}

		// <summary>
		//   Performs the explicit numeric conversions
		// </summary>
		static Expression ConvertNumericExplicit (EmitContext ec, Expression expr,
							  Type target_type)
		{
			Type expr_type = expr.Type;
			
			if (expr_type == TypeManager.sbyte_type){
				//
				// From sbyte to byte, ushort, uint, ulong, char
				//
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.uint32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U4);
				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.byte_type){
				//
				// From byte to sbyte and char
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.short_type){
				//
				// From short to sbyte, byte, ushort, uint, ulong, char
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.uint32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U4);
				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.ushort_type){
				//
				// From ushort to sbyte, byte, short, char
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.int32_type){
				//
				// From int to sbyte, byte, short, ushort, uint, ulong, char
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.uint32_type)
					return new EmptyCast (expr, target_type);
				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.uint32_type){
				//
				// From uint to sbyte, byte, short, ushort, int, char
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.int32_type)
					return new EmptyCast (expr, target_type);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.int64_type){
				//
				// From long to sbyte, byte, short, ushort, int, uint, ulong, char
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.int32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I4);
				if (target_type == TypeManager.uint32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U4);
				if (target_type == TypeManager.uint64_type)
					return new EmptyCast (expr, target_type);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.uint64_type){
				//
				// From ulong to sbyte, byte, short, ushort, int, uint, long, char
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.int32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I4);
				if (target_type == TypeManager.uint32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U4);
				if (target_type == TypeManager.int64_type)
					return new EmptyCast (expr, target_type);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
			} else if (expr_type == TypeManager.char_type){
				//
				// From char to sbyte, byte, short
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
			} else if (expr_type == TypeManager.float_type){
				//
				// From float to sbyte, byte, short,
				// ushort, int, uint, long, ulong, char
				// or decimal
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.int32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I4);
				if (target_type == TypeManager.uint32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U4);
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U8);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} else if (expr_type == TypeManager.double_type){
				//
				// From double to byte, byte, short,
				// ushort, int, uint, long, ulong,
				// char, float or decimal
				//
				if (target_type == TypeManager.sbyte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I1);
				if (target_type == TypeManager.byte_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U1);
				if (target_type == TypeManager.short_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I2);
				if (target_type == TypeManager.ushort_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.int32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I4);
				if (target_type == TypeManager.uint32_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U4);
				if (target_type == TypeManager.int64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_I8);
				if (target_type == TypeManager.uint64_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U8);
				if (target_type == TypeManager.char_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_U2);
				if (target_type == TypeManager.float_type)
					return new OpcodeCast (expr, target_type, OpCodes.Conv_R4);
				if (target_type == TypeManager.decimal_type)
					return InternalTypeConstructor (ec, expr, target_type);
			} 

			// decimal is taken care of by the op_Explicit methods.

			return null;
		}

		// <summary>
		//  Returns whether an explicit reference conversion can be performed
		//  from source_type to target_type
		// </summary>
		static bool ExplicitReferenceConversionExists (Type source_type, Type target_type)
		{
			bool target_is_value_type = target_type.IsValueType;
			
			if (source_type == target_type)
				return true;
			
			//
			// From object to any reference type
			//
			if (source_type == TypeManager.object_type && !target_is_value_type)
				return true;
					
			//
			// From any class S to any class-type T, provided S is a base class of T
			//
			if (target_type.IsSubclassOf (source_type))
				return true;

			//
			// From any interface type S to any interface T provided S is not derived from T
			//
			if (source_type.IsInterface && target_type.IsInterface){
				if (!target_type.IsSubclassOf (source_type))
					return true;
			}
			    
			//
			// From any class type S to any interface T, provides S is not sealed
			// and provided S does not implement T.
			//
			if (target_type.IsInterface && !source_type.IsSealed &&
			    !target_type.IsAssignableFrom (source_type))
				return true;

			//
			// From any interface-type S to to any class type T, provided T is not
			// sealed, or provided T implements S.
			//
			if (source_type.IsInterface &&
			    (!target_type.IsSealed || source_type.IsAssignableFrom (target_type)))
				return true;

			// From an array type S with an element type Se to an array type T with an 
			// element type Te provided all the following are true:
			//     * S and T differe only in element type, in other words, S and T
			//       have the same number of dimensions.
			//     * Both Se and Te are reference types
			//     * An explicit referenc conversions exist from Se to Te
			//
			if (source_type.IsArray && target_type.IsArray) {
				if (source_type.GetArrayRank () == target_type.GetArrayRank ()) {
					
					Type source_element_type = source_type.GetElementType ();
					Type target_element_type = target_type.GetElementType ();
					
					if (!source_element_type.IsValueType && !target_element_type.IsValueType)
						if (ExplicitReferenceConversionExists (source_element_type,
										       target_element_type))
							return true;
				}
			}
			

			// From System.Array to any array-type
			if (source_type == TypeManager.array_type &&
			    target_type.IsSubclassOf (TypeManager.array_type)){
				return true;
			}

			//
			// From System delegate to any delegate-type
			//
			if (source_type == TypeManager.delegate_type &&
			    target_type.IsSubclassOf (TypeManager.delegate_type))
				return true;

			//
			// From ICloneable to Array or Delegate types
			//
			if (source_type == TypeManager.icloneable_type &&
			    (target_type == TypeManager.array_type ||
			     target_type == TypeManager.delegate_type))
				return true;
			
			return false;
		}

		// <summary>
		//   Implements Explicit Reference conversions
		// </summary>
		static Expression ConvertReferenceExplicit (Expression source, Type target_type)
		{
			Type source_type = source.Type;
			bool target_is_value_type = target_type.IsValueType;
			
			//
			// From object to any reference type
			//
			if (source_type == TypeManager.object_type && !target_is_value_type)
				return new ClassCast (source, target_type);


			//
			// From any class S to any class-type T, provided S is a base class of T
			//
			if (target_type.IsSubclassOf (source_type))
				return new ClassCast (source, target_type);

			//
			// From any interface type S to any interface T provided S is not derived from T
			//
			if (source_type.IsInterface && target_type.IsInterface){

				Type [] ifaces = source_type.GetInterfaces ();

				if (TypeManager.ImplementsInterface (source_type, target_type))
					return null;
				else
					return new ClassCast (source, target_type);
			}
			    
			//
			// From any class type S to any interface T, provides S is not sealed
			// and provided S does not implement T.
			//
			if (target_type.IsInterface && !source_type.IsSealed) {
				
				if (TypeManager.ImplementsInterface (source_type, target_type))
					return null;
				else
					return new ClassCast (source, target_type);
				
			}

			//
			// From any interface-type S to to any class type T, provided T is not
			// sealed, or provided T implements S.
			//
			if (source_type.IsInterface) {

				if (target_type.IsSealed)
					return null;
				
				if (TypeManager.ImplementsInterface (target_type, source_type))
					return new ClassCast (source, target_type);
				else
					return null;
			}
			
			// From an array type S with an element type Se to an array type T with an 
			// element type Te provided all the following are true:
			//     * S and T differe only in element type, in other words, S and T
			//       have the same number of dimensions.
			//     * Both Se and Te are reference types
			//     * An explicit referenc conversions exist from Se to Te
			//
			if (source_type.IsArray && target_type.IsArray) {
				if (source_type.GetArrayRank () == target_type.GetArrayRank ()) {
					
					Type source_element_type = source_type.GetElementType ();
					Type target_element_type = target_type.GetElementType ();
					
					if (!source_element_type.IsValueType && !target_element_type.IsValueType)
						if (ExplicitReferenceConversionExists (source_element_type,
										       target_element_type))
							return new ClassCast (source, target_type);
				}
			}
			

			// From System.Array to any array-type
			if (source_type == TypeManager.array_type &&
			    target_type.IsSubclassOf (TypeManager.array_type)){
				return new ClassCast (source, target_type);
			}

			//
			// From System delegate to any delegate-type
			//
			if (source_type == TypeManager.delegate_type &&
			    target_type.IsSubclassOf (TypeManager.delegate_type))
				return new ClassCast (source, target_type);

			//
			// From ICloneable to Array or Delegate types
			//
			if (source_type == TypeManager.icloneable_type &&
			    (target_type == TypeManager.array_type ||
			     target_type == TypeManager.delegate_type))
				return new ClassCast (source, target_type);
			
			return null;
		}
		
		// <summary>
		//   Performs an explicit conversion of the expression `expr' whose
		//   type is expr.Type to `target_type'.
		// </summary>
		static public Expression ConvertExplicit (EmitContext ec, Expression expr,
							  Type target_type, Location loc)
		{
			Expression ne = ConvertImplicitStandard (ec, expr, target_type, loc);

			if (ne != null)
				return ne;

			ne = ConvertNumericExplicit (ec, expr, target_type);
			if (ne != null)
				return ne;

			ne = ConvertReferenceExplicit (expr, target_type);
			if (ne != null)
				return ne;

			ne = ExplicitUserConversion (ec, expr, target_type, loc);
			if (ne != null)
				return ne;

			Report.Error (30, loc, "Cannot convert type '" + TypeManager.CSharpName (expr.Type) + "' to '"
				      + TypeManager.CSharpName (target_type) + "'");
			return null;
		}

		// <summary>
		//   Same as ConverExplicit, only it doesn't include user defined conversions
		// </summary>
		static public Expression ConvertExplicitStandard (EmitContext ec, Expression expr,
								  Type target_type, Location l)
		{
			Expression ne = ConvertImplicitStandard (ec, expr, target_type, l);

			if (ne != null)
				return ne;

			ne = ConvertNumericExplicit (ec, expr, target_type);
			if (ne != null)
				return ne;

			ne = ConvertReferenceExplicit (expr, target_type);
			if (ne != null)
				return ne;

			Report.Error (30, l, "Cannot convert type '" +
				      TypeManager.CSharpName (expr.Type) + "' to '" + 
				      TypeManager.CSharpName (target_type) + "'");
			return null;
		}

		static string ExprClassName (ExprClass c)
		{
			switch (c){
			case ExprClass.Invalid:
				return "Invalid";
			case ExprClass.Value:
				return "value";
			case ExprClass.Variable:
				return "variable";
			case ExprClass.Namespace:
				return "namespace";
			case ExprClass.Type:
				return "type";
			case ExprClass.MethodGroup:
				return "method group";
			case ExprClass.PropertyAccess:
				return "property access";
			case ExprClass.EventAccess:
				return "event access";
			case ExprClass.IndexerAccess:
				return "indexer access";
			case ExprClass.Nothing:
				return "null";
			}
			throw new Exception ("Should not happen");
		}
		
		// <summary>
		//   Reports that we were expecting `expr' to be of class `expected'
		// </summary>
		protected void report118 (Location loc, Expression expr, string expected)
		{
			string kind = "Unknown";
			
			if (expr != null)
				kind = ExprClassName (expr.ExprClass);

			Error (118, loc, "Expression denotes a '" + kind +
			       "' where an " + expected + " was expected");
		}

		// <summary>
		//   This function tries to reduce the expression performing
		//   constant folding and common subexpression elimination
		// </summary>
		static public Expression Reduce (EmitContext ec, Expression e)
		{
			//Console.WriteLine ("Calling reduce");
			return e.Reduce (ec);
		}
	}

	// <summary>
	//   This is just a base class for expressions that can
	//   appear on statements (invocations, object creation,
	//   assignments, post/pre increment and decrement).  The idea
	//   being that they would support an extra Emition interface that
	//   does not leave a result on the stack.
	// </summary>

	public abstract class ExpressionStatement : Expression {

		// <summary>
		//   Requests the expression to be emitted in a `statement'
		//   context.  This means that no new value is left on the
		//   stack after invoking this method (constrasted with
		//   Emit that will always leave a value on the stack).
		// </summary>
		public abstract void EmitStatement (EmitContext ec);
	}

	// <summary>
	//   This kind of cast is used to encapsulate the child
	//   whose type is child.Type into an expression that is
	//   reported to return "return_type".  This is used to encapsulate
	//   expressions which have compatible types, but need to be dealt
	//   at higher levels with.
	//
	//   For example, a "byte" expression could be encapsulated in one
	//   of these as an "unsigned int".  The type for the expression
	//   would be "unsigned int".
	//
	// </summary>
	
	public class EmptyCast : Expression {
		protected Expression child;

		public EmptyCast (Expression child, Type return_type)
		{
			ExprClass = child.ExprClass;
			type = return_type;
			this.child = child;
		}

		public override Expression DoResolve (EmitContext ec)
		{
			// This should never be invoked, we are born in fully
			// initialized state.

			return this;
		}

		public override void Emit (EmitContext ec)
		{
			child.Emit (ec);
		}

	}

	// <summary>
	//  This class is used to wrap literals which belong inside Enums
	// </summary>

	public class EnumLiteral : Literal {
		Expression child;

		public EnumLiteral (Expression child, Type enum_type)
		{
			ExprClass = child.ExprClass;
			this.child = child;
			type = enum_type;
		}
		
		public override Expression DoResolve (EmitContext ec)
		{
			// This should never be invoked, we are born in fully
			// initialized state.

			return this;
		}

		public override void Emit (EmitContext ec)
		{
			child.Emit (ec);
		}

		public override object GetValue ()
		{
			return ((Literal) child).GetValue ();
		}

		public override string AsString ()
		{
			return ((Literal) child).AsString ();
		}
	}

	// <summary>
	//   This kind of cast is used to encapsulate Value Types in objects.
	//
	//   The effect of it is to box the value type emitted by the previous
	//   operation.
	// </summary>
	public class BoxedCast : EmptyCast {

		public BoxedCast (Expression expr)
			: base (expr, TypeManager.object_type)
		{
		}

		public override Expression DoResolve (EmitContext ec)
		{
			// This should never be invoked, we are born in fully
			// initialized state.

			return this;
		}

		public override void Emit (EmitContext ec)
		{
			base.Emit (ec);
			ec.ig.Emit (OpCodes.Box, child.Type);
		}
	}

	// <summary>
	//   This kind of cast is used to encapsulate a child expression
	//   that can be trivially converted to a target type using one or 
	//   two opcodes.  The opcodes are passed as arguments.
	// </summary>
	public class OpcodeCast : EmptyCast {
		OpCode op, op2;
		bool second_valid;
		
		public OpcodeCast (Expression child, Type return_type, OpCode op)
			: base (child, return_type)
			
		{
			this.op = op;
			second_valid = false;
		}

		public OpcodeCast (Expression child, Type return_type, OpCode op, OpCode op2)
			: base (child, return_type)
			
		{
			this.op = op;
			this.op2 = op2;
			second_valid = true;
		}

		public override Expression DoResolve (EmitContext ec)
		{
			// This should never be invoked, we are born in fully
			// initialized state.

			return this;
		}

		public override void Emit (EmitContext ec)
		{
			base.Emit (ec);
			ec.ig.Emit (op);

			if (second_valid)
				ec.ig.Emit (op2);
		}			
		
	}

	// <summary>
	//   This kind of cast is used to encapsulate a child and cast it
	//   to the class requested
	// </summary>
	public class ClassCast : EmptyCast {
		public ClassCast (Expression child, Type return_type)
			: base (child, return_type)
			
		{
		}

		public override Expression DoResolve (EmitContext ec)
		{
			// This should never be invoked, we are born in fully
			// initialized state.

			return this;
		}

		public override void Emit (EmitContext ec)
		{
			base.Emit (ec);

			ec.ig.Emit (OpCodes.Castclass, type);
		}			
		
	}
	
	//
	// SimpleName expressions are initially formed of a single
	// word and it only happens at the beginning of the expression.
	//
	// The expression will try to be bound to a Field, a Method
	// group or a Property.  If those fail we pass the name to our
	// caller and the SimpleName is compounded to perform a type
	// lookup.  The idea behind this process is that we want to avoid
	// creating a namespace map from the assemblies, as that requires
	// the GetExportedTypes function to be called and a hashtable to
	// be constructed which reduces startup time.  If later we find
	// that this is slower, we should create a `NamespaceExpr' expression
	// that fully participates in the resolution process. 
	//
	// For example `System.Console.WriteLine' is decomposed into
	// MemberAccess (MemberAccess (SimpleName ("System"), "Console"), "WriteLine")
	//
	// The first SimpleName wont produce a match on its own, so it will
	// be turned into:
	// MemberAccess (SimpleName ("System.Console"), "WriteLine").
	//
	// System.Console will produce a TypeExpr match.
	//
	// The downside of this is that we might be hitting `LookupType' too many
	// times with this scheme.
	//
	public class SimpleName : Expression {
		public readonly string Name;
		public readonly Location Location;
		
		public SimpleName (string name, Location l)
		{
			Name = name;
			Location = l;
		}

		public static void Error120 (Location l, string name)
		{
			Report.Error (
				120, l,
				"An object reference is required " +
				"for the non-static field `"+name+"'");
		}
		
		//
		// Checks whether we are trying to access an instance
		// property, method or field from a static body.
		//
		Expression MemberStaticCheck (Expression e)
		{
			if (e is FieldExpr){
				FieldInfo fi = ((FieldExpr) e).FieldInfo;
				
				if (!fi.IsStatic){
					Error120 (Location, Name);
					return null;
				}
			} else if (e is MethodGroupExpr){
				MethodGroupExpr mg = (MethodGroupExpr) e;

				if (!mg.RemoveInstanceMethods ()){
					Error120 (Location, mg.Methods [0].Name);
					return null;
				}
				return e;
			} else if (e is PropertyExpr){
				if (!((PropertyExpr) e).IsStatic){
					Error120 (Location, Name);
					return null;
				}
			}

			return e;
		}
		
		//
		// 7.5.2: Simple Names. 
		//
		// Local Variables and Parameters are handled at
		// parse time, so they never occur as SimpleNames.
		//
		public override Expression DoResolve (EmitContext ec)
		{
			Expression e;

			//
			// Stage 1: Performed by the parser (binding to local or parameters).
			//

			//
			// Stage 2: Lookup members
			//
			e = MemberLookup (ec, ec.TypeContainer.TypeBuilder, Name, true, Location);
			if (e == null){
				//
				// Stage 3: Lookup symbol in the various namespaces. 
				// 
				Type t;
				
				if ((t = ec.TypeContainer.LookupType (Name, true)) != null)
					return new TypeExpr (t);

				//
				// Stage 3 part b: Lookup up if we are an alias to a type
				// or a namespace.
				//
				// Since we are cheating: we only do the Alias lookup for
				// namespaces if the name does not include any dots in it
				//
				
				// IMPLEMENT ME.  Read mcs/mcs/TODO for ideas, or rewrite
				// using NamespaceExprs (dunno how that fixes the alias
				// per-file though).
				
				// No match, maybe our parent can compose us
				// into something meaningful.
				//
				return this;
			}

			// Step 2, continues here.
			if (e is TypeExpr)
				return e;

			if (e is FieldExpr){
				FieldExpr fe = (FieldExpr) e;
				
				if (!fe.FieldInfo.IsStatic)
					fe.InstanceExpression = new This (Location.Null);
			} 				

			if (ec.IsStatic)
				return MemberStaticCheck (e);
			else
				return e;
		}

		public override void Emit (EmitContext ec)
		{
			//
			// If this is ever reached, then we failed to
			// find the name as a namespace
			//

			Error (103, Location, "The name `" + Name +
			       "' does not exist in the class `" +
			       ec.TypeContainer.Name + "'");
		}
	}
	
	// <summary>
	//   Fully resolved expression that evaluates to a type
	// </summary>
	public class TypeExpr : Expression {
		public TypeExpr (Type t)
		{
			Type = t;
			eclass = ExprClass.Type;
		}

		override public Expression DoResolve (EmitContext ec)
		{
			return this;
		}

		override public void Emit (EmitContext ec)
		{
			throw new Exception ("Implement me");
		}
	}

	// <summary>
	//   MethodGroup Expression.
	//  
	//   This is a fully resolved expression that evaluates to a type
	// </summary>
	public class MethodGroupExpr : Expression {
		public MethodBase [] Methods;
		Expression instance_expression = null;
		
		public MethodGroupExpr (MemberInfo [] mi)
		{
			Methods = new MethodBase [mi.Length];
			mi.CopyTo (Methods, 0);
			eclass = ExprClass.MethodGroup;
		}

		public MethodGroupExpr (ArrayList l)
		{
			Methods = new MethodBase [l.Count];

			l.CopyTo (Methods, 0);
			eclass = ExprClass.MethodGroup;
		}
		
		//
		// `A method group may have associated an instance expression' 
		// 
		public Expression InstanceExpression {
			get {
				return instance_expression;
			}

			set {
				instance_expression = value;
			}
		}
		
		override public Expression DoResolve (EmitContext ec)
		{
			return this;
		}

		override public void Emit (EmitContext ec)
		{
			throw new Exception ("This should never be reached");
		}

		bool RemoveMethods (bool keep_static)
		{
			ArrayList smethods = new ArrayList ();
			int top = Methods.Length;
			int i;
			
			for (i = 0; i < top; i++){
				MethodBase mb = Methods [i];

				if (mb.IsStatic == keep_static)
					smethods.Add (mb);
			}

			if (smethods.Count == 0)
				return false;

			Methods = new MethodBase [smethods.Count];
			smethods.CopyTo (Methods, 0);

			return true;
		}
		
		// <summary>
		//   Removes any instance methods from the MethodGroup, returns
		//   false if the resulting set is empty.
		// </summary>
		public bool RemoveInstanceMethods ()
		{
			return RemoveMethods (true);
		}

		// <summary>
		//   Removes any static methods from the MethodGroup, returns
		//   false if the resulting set is empty.
		// </summary>
		public bool RemoveStaticMethods ()
		{
			return RemoveMethods (false);
		}
	}

	// <summary>
	//   Fully resolved expression that evaluates to a Field
	// </summary>
	public class FieldExpr : Expression, IStackStore, IMemoryLocation {
		public readonly FieldInfo FieldInfo;
		public Expression InstanceExpression;
		Location loc;
		
		public FieldExpr (FieldInfo fi, Location l)
		{
			FieldInfo = fi;
			eclass = ExprClass.Variable;
			type = fi.FieldType;
			loc = l;
		}

		override public Expression DoResolve (EmitContext ec)
		{
			if (!FieldInfo.IsStatic){
				if (InstanceExpression == null){
					throw new Exception ("non-static FieldExpr without instance var\n" +
							     "You have to assign the Instance variable\n" +
							     "Of the FieldExpr to set this\n");
				}

				InstanceExpression = InstanceExpression.Resolve (ec);
				if (InstanceExpression == null)
					return null;
				
			}
			return this;
		}

		public Expression DoResolveLValue (EmitContext ec)
		{
			if (!FieldInfo.IsInitOnly)
				return this;

			//
			// InitOnly fields can only be assigned in constructors
			//

			if (ec.IsConstructor)
				return this;

			Report.Error (191, loc,
				      "Readonly field can not be assigned outside " +
				      "of constructor or variable initializer");
			
			return null;
		}

		override public void Emit (EmitContext ec)
		{
			ILGenerator ig = ec.ig;

			if (FieldInfo.IsStatic)
				ig.Emit (OpCodes.Ldsfld, FieldInfo);
			else {
				InstanceExpression.Emit (ec);
				
				ig.Emit (OpCodes.Ldfld, FieldInfo);
			}
		}

		public void Store (EmitContext ec)
		{
			if (FieldInfo.IsStatic)
				ec.ig.Emit (OpCodes.Stsfld, FieldInfo);
			else
				ec.ig.Emit (OpCodes.Stfld, FieldInfo);
		}

		public void AddressOf (EmitContext ec)
		{
			if (FieldInfo.IsStatic)
				ec.ig.Emit (OpCodes.Ldsflda, FieldInfo);
			else {
				InstanceExpression.Emit (ec);
				ec.ig.Emit (OpCodes.Ldflda, FieldInfo);
			}
		}
	}
	
	// <summary>
	//   Expression that evaluates to a Property.  The Assign class
	//   might set the `Value' expression if we are in an assignment.
	//
	//   This is not an LValue because we need to re-write the expression, we
	//   can not take data from the stack and store it.  
	// </summary>
	public class PropertyExpr : ExpressionStatement, IAssignMethod {
		public readonly PropertyInfo PropertyInfo;
		public readonly bool IsStatic;
		MethodInfo [] Accessors;
		Location loc;
		
		Expression instance_expr;
		
		public PropertyExpr (PropertyInfo pi, Location l)
		{
			PropertyInfo = pi;
			eclass = ExprClass.PropertyAccess;
			IsStatic = false;
			loc = l;
			Accessors = TypeManager.GetAccessors (pi);

			if (Accessors != null)
				for (int i = 0; i < Accessors.Length; i++){
					if (Accessors [i] != null)
						if (Accessors [i].IsStatic)
							IsStatic = true;
				}
			else
				Accessors = new MethodInfo [2];
			
			type = pi.PropertyType;
		}

		//
		// The instance expression associated with this expression
		//
		public Expression InstanceExpression {
			set {
				instance_expr = value;
			}

			get {
				return instance_expr;
			}
		}

		public bool VerifyAssignable ()
		{
			if (!PropertyInfo.CanWrite){
				Report.Error (200, loc, 
					      "The property `" + PropertyInfo.Name +
					      "' can not be assigned to, as it has not set accessor");
				return false;
			}

			return true;
		}

		override public Expression DoResolve (EmitContext ec)
		{
			if (!PropertyInfo.CanRead){
				Report.Error (154, loc, 
					      "The property `" + PropertyInfo.Name +
					      "' can not be used in " +
					      "this context because it lacks a get accessor");
				return null;
			}

			return this;
		}

		override public void Emit (EmitContext ec)
		{
			Invocation.EmitCall (ec, IsStatic, instance_expr, Accessors [0], null);
			
		}

		//
		// Implements the IAssignMethod interface for assignments
		//
		public void EmitAssign (EmitContext ec, Expression source)
		{
			Argument arg = new Argument (source, Argument.AType.Expression);
			ArrayList args = new ArrayList ();

			args.Add (arg);
			Invocation.EmitCall (ec, IsStatic, instance_expr, Accessors [1], args);
		}

		override public void EmitStatement (EmitContext ec)
		{
			Emit (ec);
			ec.ig.Emit (OpCodes.Pop);
		}
	}

	// <summary>
	//   Fully resolved expression that evaluates to a Expression
	// </summary>
	public class EventExpr : Expression {
		public readonly EventInfo EventInfo;
		Location loc;
		
		public EventExpr (EventInfo ei, Location loc)
		{
			EventInfo = ei;
			this.loc = loc;
			eclass = ExprClass.EventAccess;
		}

		override public Expression DoResolve (EmitContext ec)
		{
			// We are born in resolved state. 
			return this;
		}

		override public void Emit (EmitContext ec)
		{
			throw new Exception ("Implement me");
			// FIXME: Implement.
		}
	}
	
}