mcs/mcs/assign.cs

   1 //
   2 // assign.cs: Assignments.
   3 //
   4 // Author:
   5 //   Miguel de Icaza (miguel@ximian.com)
   6 //   Martin Baulig (martin@ximian.com)
   7 //   Marek Safar (marek.safar@gmail.com)
   8 //
   9 // Dual licensed under the terms of the MIT X11 or GNU GPL
  10 //
  11 // Copyright 2001, 2002, 2003 Ximian, Inc.
  12 // Copyright 2004-2008 Novell, Inc
  13 // Copyright 2011 Xamarin Inc
  14 //
  15 using System;
  16
  17 #if STATIC
  18 using IKVM.Reflection.Emit;
  19 #else
  20 using System.Reflection.Emit;
  21 #endif
  22
  23 namespace Mono.CSharp {
  24
  25         /// <summary>
  26         ///   This interface is implemented by expressions that can be assigned to.
  27         /// </summary>
  28         /// <remarks>
  29         ///   This interface is implemented by Expressions whose values can not
  30         ///   store the result on the top of the stack.
  31         ///
  32         ///   Expressions implementing this (Properties, Indexers and Arrays) would
  33         ///   perform an assignment of the Expression "source" into its final
  34         ///   location.
  35         ///
  36         ///   No values on the top of the stack are expected to be left by
  37         ///   invoking this method.
  38         /// </remarks>
  39         public interface IAssignMethod {
  40                 //
  41                 // This is an extra version of Emit. If leave_copy is `true'
  42                 // A copy of the expression will be left on the stack at the
  43                 // end of the code generated for EmitAssign
  44                 //
  45                 void Emit (EmitContext ec, bool leave_copy);
  46
  47                 //
  48                 // This method does the assignment
  49                 // `source' will be stored into the location specified by `this'
  50                 // if `leave_copy' is true, a copy of `source' will be left on the stack
  51                 // if `prepare_for_load' is true, when `source' is emitted, there will
  52                 // be data on the stack that it can use to compuatate its value. This is
  53                 // for expressions like a [f ()] ++, where you can't call `f ()' twice.
  54                 //
  55                 void EmitAssign (EmitContext ec, Expression source, bool leave_copy, bool isCompound);
  56
  57                 /*
  58                 For simple assignments, this interface is very simple, EmitAssign is called with source
  59                 as the source expression and leave_copy and prepare_for_load false.
  60
  61                 For compound assignments it gets complicated.
  62
  63                 EmitAssign will be called as before, however, prepare_for_load will be
  64                 true. The @source expression will contain an expression
  65                 which calls Emit. So, the calls look like:
  66
  67                 this.EmitAssign (ec, source, false, true) ->
  68                         source.Emit (ec); ->
  69                                 [...] ->
  70                                         this.Emit (ec, false); ->
  71                                         end this.Emit (ec, false); ->
  72                                 end [...]
  73                         end source.Emit (ec);
  74                 end this.EmitAssign (ec, source, false, true)
  75
  76
  77                 When prepare_for_load is true, EmitAssign emits a `token' on the stack that
  78                 Emit will use for its state.
  79
  80                 Let's take FieldExpr as an example. assume we are emitting f ().y += 1;
  81
  82                 Here is the call tree again. This time, each call is annotated with the IL
  83                 it produces:
  84
  85                 this.EmitAssign (ec, source, false, true)
  86                         call f
  87                         dup
  88
  89                         Binary.Emit ()
  90                                 this.Emit (ec, false);
  91                                 ldfld y
  92                                 end this.Emit (ec, false);
  93
  94                                 IntConstant.Emit ()
  95                                 ldc.i4.1
  96                                 end IntConstant.Emit
  97
  98                                 add
  99                         end Binary.Emit ()
 100
 101                         stfld
 102                 end this.EmitAssign (ec, source, false, true)
 103
 104                 Observe two things:
 105                         1) EmitAssign left a token on the stack. It was the result of f ().
 106                         2) This token was used by Emit
 107
 108                 leave_copy (in both EmitAssign and Emit) tells the compiler to leave a copy
 109                 of the expression at that point in evaluation. This is used for pre/post inc/dec
 110                 and for a = x += y. Let's do the above example with leave_copy true in EmitAssign
 111
 112                 this.EmitAssign (ec, source, true, true)
 113                         call f
 114                         dup
 115
 116                         Binary.Emit ()
 117                                 this.Emit (ec, false);
 118                                 ldfld y
 119                                 end this.Emit (ec, false);
 120
 121                                 IntConstant.Emit ()
 122                                 ldc.i4.1
 123                                 end IntConstant.Emit
 124
 125                                 add
 126                         end Binary.Emit ()
 127
 128                         dup
 129                         stloc temp
 130                         stfld
 131                         ldloc temp
 132                 end this.EmitAssign (ec, source, true, true)
 133
 134                 And with it true in Emit
 135
 136                 this.EmitAssign (ec, source, false, true)
 137                         call f
 138                         dup
 139
 140                         Binary.Emit ()
 141                                 this.Emit (ec, true);
 142                                 ldfld y
 143                                 dup
 144                                 stloc temp
 145                                 end this.Emit (ec, true);
 146
 147                                 IntConstant.Emit ()
 148                                 ldc.i4.1
 149                                 end IntConstant.Emit
 150
 151                                 add
 152                         end Binary.Emit ()
 153
 154                         stfld
 155                         ldloc temp
 156                 end this.EmitAssign (ec, source, false, true)
 157
 158                 Note that these two examples are what happens for ++x and x++, respectively.
 159                 */
 160         }
 161
 162         /// <summary>
 163         ///   An Expression to hold a temporary value.
 164         /// </summary>
 165         /// <remarks>
 166         ///   The LocalTemporary class is used to hold temporary values of a given
 167         ///   type to "simulate" the expression semantics. The local variable is
 168         ///   never captured.
 169         ///
 170         ///   The local temporary is used to alter the normal flow of code generation
 171         ///   basically it creates a local variable, and its emit instruction generates
 172         ///   code to access this value, return its address or save its value.
 173         ///
 174         ///   If `is_address' is true, then the value that we store is the address to the
 175         ///   real value, and not the value itself.
 176         ///
 177         ///   This is needed for a value type, because otherwise you just end up making a
 178         ///   copy of the value on the stack and modifying it. You really need a pointer
 179         ///   to the origional value so that you can modify it in that location. This
 180         ///   Does not happen with a class because a class is a pointer -- so you always
 181         ///   get the indirection.
 182         ///
 183         /// </remarks>
 184         public class LocalTemporary : Expression, IMemoryLocation, IAssignMethod {
 185                 LocalBuilder builder;
 186
 187                 public LocalTemporary (TypeSpec t)
 188                 {
 189                         type = t;
 190                         eclass = ExprClass.Value;
 191                 }
 192
 193                 public LocalTemporary (LocalBuilder b, TypeSpec t)
 194                         : this (t)
 195                 {
 196                         builder = b;
 197                 }
 198
 199                 public void Release (EmitContext ec)
 200                 {
 201                         ec.FreeTemporaryLocal (builder, type);
 202                         builder = null;
 203                 }
 204
 205                 public override bool ContainsEmitWithAwait ()
 206                 {
 207                         return false;
 208                 }
 209
 210                 public override Expression CreateExpressionTree (ResolveContext ec)
 211                 {
 212                         Arguments args = new Arguments (1);
 213                         args.Add (new Argument (this));
 214                         return CreateExpressionFactoryCall (ec, "Constant", args);
 215                 }
 216
 217                 protected override Expression DoResolve (ResolveContext ec)
 218                 {
 219                         return this;
 220                 }
 221
 222                 public override Expression DoResolveLValue (ResolveContext ec, Expression right_side)
 223                 {
 224                         return this;
 225                 }
 226
 227                 public override void Emit (EmitContext ec)
 228                 {
 229                         if (builder == null)
 230                                 throw new InternalErrorException ("Emit without Store, or after Release");
 231
 232                         ec.Emit (OpCodes.Ldloc, builder);
 233                 }
 234
 235                 #region IAssignMethod Members
 236
 237                 public void Emit (EmitContext ec, bool leave_copy)
 238                 {
 239                         Emit (ec);
 240
 241                         if (leave_copy)
 242                                 Emit (ec);
 243                 }
 244
 245                 public void EmitAssign (EmitContext ec, Expression source, bool leave_copy, bool isCompound)
 246                 {
 247                         if (isCompound)
 248                                 throw new NotImplementedException ();
 249
 250                         source.Emit (ec);
 251
 252                         Store (ec);
 253
 254                         if (leave_copy)
 255                                 Emit (ec);
 256                 }
 257
 258                 #endregion
 259
 260                 public LocalBuilder Builder {
 261                         get { return builder; }
 262                 }
 263
 264                 public void Store (EmitContext ec)
 265                 {
 266                         if (builder == null)
 267                                 builder = ec.GetTemporaryLocal (type);
 268
 269                         ec.Emit (OpCodes.Stloc, builder);
 270                 }
 271
 272                 public void AddressOf (EmitContext ec, AddressOp mode)
 273                 {
 274                         if (builder == null)
 275                                 builder = ec.GetTemporaryLocal (type);
 276
 277                         if (builder.LocalType.IsByRef) {
 278                                 //
 279                                 // if is_address, than this is just the address anyways,
 280                                 // so we just return this.
 281                                 //
 282                                 ec.Emit (OpCodes.Ldloc, builder);
 283                         } else {
 284                                 ec.Emit (OpCodes.Ldloca, builder);
 285                         }
 286                 }
 287         }
 288
 289         /// <summary>
 290         ///   The Assign node takes care of assigning the value of source into
 291         ///   the expression represented by target.
 292         /// </summary>
 293         public abstract class Assign : ExpressionStatement {
 294                 protected Expression target, source;
 295
 296                 protected Assign (Expression target, Expression source, Location loc)
 297                 {
 298                         this.target = target;
 299                         this.source = source;
 300                         this.loc = loc;
 301                 }
 302
 303                 public Expression Target {
 304                         get { return target; }
 305                 }
 306
 307                 public Expression Source {
 308                         get {
 309                                 return source;
 310                         }
 311                 }
 312
 313                 public override bool ContainsEmitWithAwait ()
 314                 {
 315                         return target.ContainsEmitWithAwait () || source.ContainsEmitWithAwait ();
 316                 }
 317
 318                 public override Expression CreateExpressionTree (ResolveContext ec)
 319                 {
 320                         ec.Report.Error (832, loc, "An expression tree cannot contain an assignment operator");
 321                         return null;
 322                 }
 323
 324                 protected override Expression DoResolve (ResolveContext ec)
 325                 {
 326                         bool ok = true;
 327                         source = source.Resolve (ec);
 328
 329                         if (source == null) {
 330                                 ok = false;
 331                                 source = EmptyExpression.Null;
 332                         }
 333
 334                         target = target.ResolveLValue (ec, source);
 335
 336                         if (target == null || !ok)
 337                                 return null;
 338
 339                         TypeSpec target_type = target.Type;
 340                         TypeSpec source_type = source.Type;
 341
 342                         eclass = ExprClass.Value;
 343                         type = target_type;
 344
 345                         if (!(target is IAssignMethod)) {
 346                                 Error_ValueAssignment (ec, source);
 347                                 return null;
 348                         }
 349
 350                         if (target_type != source_type) {
 351                                 Expression resolved = ResolveConversions (ec);
 352
 353                                 if (resolved != this)
 354                                         return resolved;
 355                         }
 356
 357                         return this;
 358                 }
 359
 360 #if NET_4_0
 361                 public override System.Linq.Expressions.Expression MakeExpression (BuilderContext ctx)
 362                 {
 363                         var tassign = target as IDynamicAssign;
 364                         if (tassign == null)
 365                                 throw new InternalErrorException (target.GetType () + " does not support dynamic assignment");
 366
 367                         var target_object = tassign.MakeAssignExpression (ctx, source);
 368
 369                         //
 370                         // Some hacking is needed as DLR does not support void type and requires
 371                         // always have object convertible return type to support caching and chaining
 372                         //
 373                         // We do this by introducing an explicit block which returns RHS value when
 374                         // available or null
 375                         //
 376                         if (target_object.NodeType == System.Linq.Expressions.ExpressionType.Block)
 377                                 return target_object;
 378
 379                         System.Linq.Expressions.UnaryExpression source_object;
 380                         if (ctx.HasSet (BuilderContext.Options.CheckedScope)) {
 381                                 source_object = System.Linq.Expressions.Expression.ConvertChecked (source.MakeExpression (ctx), target_object.Type);
 382                         } else {
 383                                 source_object = System.Linq.Expressions.Expression.Convert (source.MakeExpression (ctx), target_object.Type);
 384                         }
 385
 386                         return System.Linq.Expressions.Expression.Assign (target_object, source_object);
 387                 }
 388 #endif
 389                 protected virtual Expression ResolveConversions (ResolveContext ec)
 390                 {
 391                         source = Convert.ImplicitConversionRequired (ec, source, target.Type, source.Location);
 392                         if (source == null)
 393                                 return null;
 394
 395                         return this;
 396                 }
 397
 398                 void Emit (EmitContext ec, bool is_statement)
 399                 {
 400                         IAssignMethod t = (IAssignMethod) target;
 401                         t.EmitAssign (ec, source, !is_statement, this is CompoundAssign);
 402                 }
 403
 404                 public override void Emit (EmitContext ec)
 405                 {
 406                         Emit (ec, false);
 407                 }
 408
 409                 public override void EmitStatement (EmitContext ec)
 410                 {
 411                         Emit (ec, true);
 412                 }
 413
 414                 protected override void CloneTo (CloneContext clonectx, Expression t)
 415                 {
 416                         Assign _target = (Assign) t;
 417
 418                         _target.target = target.Clone (clonectx);
 419                         _target.source = source.Clone (clonectx);
 420                 }
 421
 422                 public override object Accept (StructuralVisitor visitor)
 423                 {
 424                         return visitor.Visit (this);
 425                 }
 426         }
 427
 428         public class SimpleAssign : Assign
 429         {
 430                 public SimpleAssign (Expression target, Expression source)
 431                         : this (target, source, target.Location)
 432                 {
 433                 }
 434
 435                 public SimpleAssign (Expression target, Expression source, Location loc)
 436                         : base (target, source, loc)
 437                 {
 438                 }
 439
 440                 bool CheckEqualAssign (Expression t)
 441                 {
 442                         if (source is Assign) {
 443                                 Assign a = (Assign) source;
 444                                 if (t.Equals (a.Target))
 445                                         return true;
 446                                 return a is SimpleAssign && ((SimpleAssign) a).CheckEqualAssign (t);
 447                         }
 448                         return t.Equals (source);
 449                 }
 450
 451                 protected override Expression DoResolve (ResolveContext ec)
 452                 {
 453                         Expression e = base.DoResolve (ec);
 454                         if (e == null || e != this)
 455                                 return e;
 456
 457                         if (CheckEqualAssign (target))
 458                                 ec.Report.Warning (1717, 3, loc, "Assignment made to same variable; did you mean to assign something else?");
 459
 460                         return this;
 461                 }
 462         }
 463
 464         public class RuntimeExplicitAssign : Assign
 465         {
 466                 public RuntimeExplicitAssign (Expression target, Expression source)
 467                         : base (target, source, target.Location)
 468                 {
 469                 }
 470
 471                 protected override Expression ResolveConversions (ResolveContext ec)
 472                 {
 473                         source = EmptyCast.Create (source, target.Type);
 474                         return this;
 475                 }
 476         }
 477
 478         //
 479         // Compiler generated assign
 480         //
 481         class CompilerAssign : Assign
 482         {
 483                 public CompilerAssign (Expression target, Expression source, Location loc)
 484                         : base (target, source, loc)
 485                 {
 486                 }
 487
 488                 protected override Expression DoResolve (ResolveContext ec)
 489                 {
 490                         var expr = base.DoResolve (ec);
 491                         var vr = target as VariableReference;
 492                         if (vr != null && vr.VariableInfo != null)
 493                                 vr.VariableInfo.IsEverAssigned = false;
 494
 495                         return expr;
 496                 }
 497
 498                 public void UpdateSource (Expression source)
 499                 {
 500                         base.source = source;
 501                 }
 502         }
 503
 504         //
 505         // Implements fields and events class initializers
 506         //
 507         public class FieldInitializer : Assign
 508         {
 509                 //
 510                 // Field initializers are tricky for partial classes. They have to
 511                 // share same constructor (block) for expression trees resolve but
 512                 // they have they own resolve scope
 513                 //
 514                 sealed class FieldInitializerContext : ResolveContext
 515                 {
 516                         ExplicitBlock ctor_block;
 517
 518                         public FieldInitializerContext (IMemberContext mc, ResolveContext constructorContext)
 519                                 : base (mc, Options.FieldInitializerScope | Options.ConstructorScope)
 520                         {
 521                                 this.ctor_block = constructorContext.CurrentBlock.Explicit;
 522                         }
 523
 524                         public override ExplicitBlock ConstructorBlock {
 525                                 get {
 526                                         return ctor_block;
 527                                 }
 528                         }
 529                 }
 530
 531                 //
 532                 // Keep resolved value because field initializers have their own rules
 533                 //
 534                 ExpressionStatement resolved;
 535                 IMemberContext mc;
 536
 537                 public FieldInitializer (FieldSpec spec, Expression expression, IMemberContext mc)
 538                         : base (new FieldExpr (spec, expression.Location), expression, expression.Location)
 539                 {
 540                         this.mc = mc;
 541                         if (!spec.IsStatic)
 542                                 ((FieldExpr)target).InstanceExpression = new CompilerGeneratedThis (mc.CurrentType, expression.Location);
 543                 }
 544
 545                 protected override Expression DoResolve (ResolveContext ec)
 546                 {
 547                         // Field initializer can be resolved (fail) many times
 548                         if (source == null)
 549                                 return null;
 550
 551                         if (resolved == null) {
 552                                 var ctx = new FieldInitializerContext (mc, ec);
 553                                 resolved = base.DoResolve (ctx) as ExpressionStatement;
 554                         }
 555
 556                         return resolved;
 557                 }
 558
 559                 public override void EmitStatement (EmitContext ec)
 560                 {
 561                         if (resolved == null)
 562                                 return;
 563
 564                         if (resolved != this)
 565                                 resolved.EmitStatement (ec);
 566                         else
 567                                 base.EmitStatement (ec);
 568                 }
 569
 570                 public bool IsDefaultInitializer {
 571                         get {
 572                                 Constant c = source as Constant;
 573                                 if (c == null)
 574                                         return false;
 575
 576                                 FieldExpr fe = (FieldExpr)target;
 577                                 return c.IsDefaultInitializer (fe.Type);
 578                         }
 579                 }
 580
 581                 public override bool IsSideEffectFree {
 582                         get {
 583                                 return source.IsSideEffectFree;
 584                         }
 585                 }
 586         }
 587
 588         //
 589         // This class is used for compound assignments.
 590         //
 591         public class CompoundAssign : Assign
 592         {
 593                 // This is just a hack implemented for arrays only
 594                 public sealed class TargetExpression : Expression
 595                 {
 596                         readonly Expression child;
 597
 598                         public TargetExpression (Expression child)
 599                         {
 600                                 this.child = child;
 601                                 this.loc = child.Location;
 602                         }
 603
 604                         public override bool ContainsEmitWithAwait ()
 605                         {
 606                                 return child.ContainsEmitWithAwait ();
 607                         }
 608
 609                         public override Expression CreateExpressionTree (ResolveContext ec)
 610                         {
 611                                 throw new NotSupportedException ("ET");
 612                         }
 613
 614                         protected override Expression DoResolve (ResolveContext ec)
 615                         {
 616                                 type = child.Type;
 617                                 eclass = ExprClass.Value;
 618                                 return this;
 619                         }
 620
 621                         public override void Emit (EmitContext ec)
 622                         {
 623                                 child.Emit (ec);
 624                         }
 625
 626                         public override Expression EmitToField (EmitContext ec)
 627                         {
 628                                 return child.EmitToField (ec);
 629                         }
 630                 }
 631
 632                 // Used for underlying binary operator
 633                 readonly Binary.Operator op;
 634                 Expression right;
 635                 Expression left;
 636
 637                 public CompoundAssign (Binary.Operator op, Expression target, Expression source, Location loc)
 638                         : base (target, source, loc)
 639                 {
 640                         right = source;
 641                         this.op = op;
 642                 }
 643
 644                 public CompoundAssign (Binary.Operator op, Expression target, Expression source, Expression left, Location loc)
 645                         : this (op, target, source, loc)
 646                 {
 647                         this.left = left;
 648                 }
 649
 650                 public Binary.Operator Operator {
 651                         get {
 652                                 return op;
 653                         }
 654                 }
 655
 656                 protected override Expression DoResolve (ResolveContext ec)
 657                 {
 658                         right = right.Resolve (ec);
 659                         if (right == null)
 660                                 return null;
 661
 662                         MemberAccess ma = target as MemberAccess;
 663                         using (ec.Set (ResolveContext.Options.CompoundAssignmentScope)) {
 664                                 target = target.Resolve (ec);
 665                         }
 666
 667                         if (target == null)
 668                                 return null;
 669
 670                         if (target is MethodGroupExpr){
 671                                 ec.Report.Error (1656, loc,
 672                                         "Cannot assign to `{0}' because it is a `{1}'",
 673                                         ((MethodGroupExpr)target).Name, target.ExprClassName);
 674                                 return null;
 675                         }
 676
 677                         var event_expr = target as EventExpr;
 678                         if (event_expr != null) {
 679                                 source = Convert.ImplicitConversionRequired (ec, right, target.Type, loc);
 680                                 if (source == null)
 681                                         return null;
 682
 683                                 Expression rside;
 684                                 if (op == Binary.Operator.Addition)
 685                                         rside = EmptyExpression.EventAddition;
 686                                 else if (op == Binary.Operator.Subtraction)
 687                                         rside = EmptyExpression.EventSubtraction;
 688                                 else
 689                                         rside = null;
 690
 691                                 target = target.ResolveLValue (ec, rside);
 692                                 if (target == null)
 693                                         return null;
 694
 695                                 eclass = ExprClass.Value;
 696                                 type = event_expr.Operator.ReturnType;
 697                                 return this;
 698                         }
 699
 700                         //
 701                         // Only now we can decouple the original source/target
 702                         // into a tree, to guarantee that we do not have side
 703                         // effects.
 704                         //
 705                         if (left == null)
 706                                 left = new TargetExpression (target);
 707
 708                         source = new Binary (op, left, right, true, loc);
 709
 710                         if (target is DynamicMemberAssignable) {
 711                                 Arguments targs = ((DynamicMemberAssignable) target).Arguments;
 712                                 source = source.Resolve (ec);
 713
 714                                 Arguments args = new Arguments (targs.Count + 1);
 715                                 args.AddRange (targs);
 716                                 args.Add (new Argument (source));
 717
 718                                 var binder_flags = CSharpBinderFlags.ValueFromCompoundAssignment;
 719
 720                                 //
 721                                 // Compound assignment does target conversion using additional method
 722                                 // call, set checked context as the binary operation can overflow
 723                                 //
 724                                 if (ec.HasSet (ResolveContext.Options.CheckedScope))
 725                                         binder_flags |= CSharpBinderFlags.CheckedContext;
 726
 727                                 if (target is DynamicMemberBinder) {
 728                                         source = new DynamicMemberBinder (ma.Name, binder_flags, args, loc).Resolve (ec);
 729
 730                                         // Handles possible event addition/subtraction
 731                                         if (op == Binary.Operator.Addition || op == Binary.Operator.Subtraction) {
 732                                                 args = new Arguments (targs.Count + 1);
 733                                                 args.AddRange (targs);
 734                                                 args.Add (new Argument (right));
 735                                                 string method_prefix = op == Binary.Operator.Addition ?
 736                                                         Event.AEventAccessor.AddPrefix : Event.AEventAccessor.RemovePrefix;
 737
 738                                                 var invoke = DynamicInvocation.CreateSpecialNameInvoke (
 739                                                         new MemberAccess (right, method_prefix + ma.Name, loc), args, loc).Resolve (ec);
 740
 741                                                 args = new Arguments (targs.Count);
 742                                                 args.AddRange (targs);
 743                                                 source = new DynamicEventCompoundAssign (ma.Name, args,
 744                                                         (ExpressionStatement) source, (ExpressionStatement) invoke, loc).Resolve (ec);
 745                                         }
 746                                 } else {
 747                                         source = new DynamicIndexBinder (binder_flags, args, loc).Resolve (ec);
 748                                 }
 749
 750                                 return source;
 751                         }
 752
 753                         return base.DoResolve (ec);
 754                 }
 755
 756                 protected override Expression ResolveConversions (ResolveContext ec)
 757                 {
 758                         //
 759                         // LAMESPEC: Under dynamic context no target conversion is happening
 760                         // This allows more natual dynamic behaviour but breaks compatibility
 761                         // with static binding
 762                         //
 763                         if (target is RuntimeValueExpression)
 764                                 return this;
 765
 766                         TypeSpec target_type = target.Type;
 767
 768                         //
 769                         // 1. the return type is implicitly convertible to the type of target
 770                         //
 771                         if (Convert.ImplicitConversionExists (ec, source, target_type)) {
 772                                 source = Convert.ImplicitConversion (ec, source, target_type, loc);
 773                                 return this;
 774                         }
 775
 776                         //
 777                         // Otherwise, if the selected operator is a predefined operator
 778                         //
 779                         Binary b = source as Binary;
 780                         if (b == null && source is ReducedExpression)
 781                                 b = ((ReducedExpression) source).OriginalExpression as Binary;
 782
 783                         if (b != null) {
 784                                 //
 785                                 // 2a. the operator is a shift operator
 786                                 //
 787                                 // 2b. the return type is explicitly convertible to the type of x, and
 788                                 // y is implicitly convertible to the type of x
 789                                 //
 790                                 if ((b.Oper & Binary.Operator.ShiftMask) != 0 ||
 791                                         Convert.ImplicitConversionExists (ec, right, target_type)) {
 792                                         source = Convert.ExplicitConversion (ec, source, target_type, loc);
 793                                         return this;
 794                                 }
 795                         }
 796
 797                         if (source.Type.BuiltinType == BuiltinTypeSpec.Type.Dynamic) {
 798                                 Arguments arg = new Arguments (1);
 799                                 arg.Add (new Argument (source));
 800                                 return new SimpleAssign (target, new DynamicConversion (target_type, CSharpBinderFlags.ConvertExplicit, arg, loc), loc).Resolve (ec);
 801                         }
 802
 803                         right.Error_ValueCannotBeConverted (ec, loc, target_type, false);
 804                         return null;
 805                 }
 806
 807                 protected override void CloneTo (CloneContext clonectx, Expression t)
 808                 {
 809                         CompoundAssign ctarget = (CompoundAssign) t;
 810
 811                         ctarget.right = ctarget.source = source.Clone (clonectx);
 812                         ctarget.target = target.Clone (clonectx);
 813                 }
 814
 815                 public override object Accept (StructuralVisitor visitor)
 816                 {
 817                         return visitor.Visit (this);
 818                 }
 819         }
 820 }