Allow with on anonymous types

src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs

@@ -1076,7 +1076,8 @@ private bool CheckPropertyValueKind(SyntaxNode node, BoundExpression expr, BindV
                 if (setMethod is null)
                 {
                     var containing = this.ContainingMemberOrLambda;
-                    if (!AccessingAutoPropertyFromConstructor(receiver, propertySymbol, containing))
+                    if (!AccessingAutoPropertyFromConstructor(receiver, propertySymbol, containing)
+                        && !isAllowedDespiteReadonly(receiver))
                     {
                         Error(diagnostics, ErrorCode.ERR_AssgReadonlyProp, node, propertySymbol);
                         return false;
@@ -1203,6 +1204,17 @@ bool reportUseSite(MethodSymbol accessor)
                 return false;
             }
 
+            static bool isAllowedDespiteReadonly(BoundExpression receiver)
+            {
+                // ok: anonymousType with { Property = ... }
+                if (receiver is BoundObjectOrCollectionValuePlaceholder && receiver.Type.IsAnonymousType)
+                {
+                    return true;
+                }
+
+                return false;
+            }
+
             bool isAllowedInitOnlySet(BoundExpression receiver)
             {
                 // ok: new C() { InitOnlyProperty = ... }

src/Compilers/CSharp/Portable/Binder/Binder_WithExpression.cs

@@ -32,10 +32,14 @@ private BoundExpression BindWithExpression(WithExpressionSyntax syntax, BindingD
             }
 
             MethodSymbol? cloneMethod = null;
-            if (receiverType.IsValueType)
+            if (receiverType.IsValueType && !receiverType.IsPointerOrFunctionPointer())
             {
                 CheckFeatureAvailability(syntax, MessageID.IDS_FeatureWithOnStructs, diagnostics);
             }
+            else if (receiverType.IsAnonymousType)
+            {
+                CheckFeatureAvailability(syntax, MessageID.IDS_FeatureWithOnAnonymousTypes, diagnostics);
+            }
             else if (!receiverType.IsErrorType())
             {
                 CompoundUseSiteInfo<AssemblySymbol> useSiteInfo = GetNewCompoundUseSiteInfo(diagnostics);
@@ -61,7 +65,7 @@ private BoundExpression BindWithExpression(WithExpressionSyntax syntax, BindingD
                 isForNewInstance: true,
                 diagnostics);
 
-            // N.B. Since we only don't parse nested initializers in syntax there should be no extra
+            // N.B. Since we don't parse nested initializers in syntax there should be no extra
             // errors we need to check for here.
 
             return new BoundWithExpression(

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6576,6 +6576,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureWithOnStructs" xml:space="preserve">
     <value>with on structs</value>
   </data>
+  <data name="IDS_FeatureWithOnAnonymousTypes" xml:space="preserve">
+    <value>with on anonymous types</value>
+  </data>
   <data name="IDS_FeaturePositionalFieldsInRecords" xml:space="preserve">
     <value>positional fields in records</value>
   </data>

src/Compilers/CSharp/Portable/Errors/MessageID.cs

@@ -223,6 +223,7 @@ internal enum MessageID
         IDS_FeatureGlobalUsing = MessageBase + 12798,
         IDS_FeatureInferredDelegateType = MessageBase + 12799,
         IDS_FeatureLambdaAttributes = MessageBase + 12800,
+        IDS_FeatureWithOnAnonymousTypes = MessageBase + 12801,
     }
 
     // Message IDs may refer to strings that need to be localized.
@@ -334,6 +335,7 @@ internal static LanguageVersion RequiredVersion(this MessageID feature)
                 case MessageID.IDS_FeatureSealedToStringInRecord: // semantic check
                 case MessageID.IDS_FeatureRecordStructs:
                 case MessageID.IDS_FeatureWithOnStructs: // semantic check
+                case MessageID.IDS_FeatureWithOnAnonymousTypes: // semantic check
                 case MessageID.IDS_FeaturePositionalFieldsInRecords: // semantic check
                 case MessageID.IDS_FeatureGlobalUsing:
                 case MessageID.IDS_FeatureInferredDelegateType: // semantic check

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_ObjectCreationExpression.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
@@ -103,11 +104,13 @@ public override BoundNode VisitObjectCreationExpression(BoundObjectCreationExpre
 
         public override BoundNode VisitWithExpression(BoundWithExpression withExpr)
         {
-            Debug.Assert(withExpr.Receiver.Type!.Equals(withExpr.Type, TypeCompareKind.ConsiderEverything));
+            TypeSymbol type = withExpr.Type;
+            BoundExpression receiver = withExpr.Receiver;
+            Debug.Assert(receiver.Type!.Equals(type, TypeCompareKind.ConsiderEverything));
 
             // for a with expression of the form
             //
-            //      receiver with { P1 = e1, P2 = e2 }
+            //      receiver with { P1 = e1, P2 = e2 } // P3 is copied implicitly
             //
             // if the receiver is a struct, duplicate the value, then set the given struct properties:
             //
@@ -116,6 +119,10 @@ public override BoundNode VisitWithExpression(BoundWithExpression withExpr)
             //     tmp.P2 = e2;
             //     tmp
             //
+            // if the receiver is an anonymous type, then invoke its constructor:
+            //
+            //     new Type(e1, e2, receiver.P3);
+            //
             // otherwise the receiver is a record class and we want to lower it to a call to its `Clone` method, then
             // set the given record properties. i.e.
             //
@@ -124,29 +131,80 @@ public override BoundNode VisitWithExpression(BoundWithExpression withExpr)
             //      tmp.P2 = e2;
             //      tmp
 
-            BoundExpression expression;
+            BoundExpression rewrittenReceiver = VisitExpression(receiver);
 
-            if (withExpr.Type.IsValueType)
+            if (type.IsAnonymousType)
             {
-                expression = VisitExpression(withExpr.Receiver);
+                var anonymousType = (AnonymousTypeManager.AnonymousTypePublicSymbol)type;
+                var sideEffects = ArrayBuilder<BoundExpression>.GetInstance();
+                var temps = ArrayBuilder<LocalSymbol>.GetInstance();
+                BoundLocal oldValue = _factory.StoreToTemp(rewrittenReceiver, out BoundAssignmentOperator boundAssignmentToTemp);
+                temps.Add(oldValue.LocalSymbol);
+                sideEffects.Add(boundAssignmentToTemp);
+
+                BoundExpression value = _factory.New(anonymousType, getAnonymousTypeValues(withExpr, oldValue, anonymousType, sideEffects, temps));
+
+                return new BoundSequence(withExpr.Syntax, temps.ToImmutableAndFree(), sideEffects.ToImmutableAndFree(), value, type);
+            }
+
+            BoundExpression expression;
+            if (type.IsValueType)
+            {
+                expression = rewrittenReceiver;
             }
             else
             {
                 Debug.Assert(withExpr.CloneMethod is not null);
                 Debug.Assert(withExpr.CloneMethod.ParameterCount == 0);
 
                 expression = _factory.Convert(
-                    withExpr.Type,
+                    type,
                     _factory.Call(
-                        VisitExpression(withExpr.Receiver),
+                        rewrittenReceiver,
                         withExpr.CloneMethod));
             }
 
             return MakeExpressionWithInitializer(
                 withExpr.Syntax,
                 expression,
                 withExpr.InitializerExpression,
-                withExpr.Type);
+                type);
+
+            ImmutableArray<BoundExpression> getAnonymousTypeValues(BoundWithExpression withExpr, BoundExpression oldValue, AnonymousTypeManager.AnonymousTypePublicSymbol anonymousType,
+                ArrayBuilder<BoundExpression> sideEffects, ArrayBuilder<LocalSymbol> temps)
+            {
+                var dictionary = PooledDictionary<PropertySymbol, BoundExpression>.GetInstance();
+                foreach (BoundExpression initializer in withExpr.InitializerExpression.Initializers)
+                {
+                    var assignment = (BoundAssignmentOperator)initializer;
+                    var left = (BoundObjectInitializerMember)assignment.Left;
+                    Debug.Assert(left.MemberSymbol is not null);
+
+                    // We evaluate the values provided in source first
+                    BoundLocal valueTemp = _factory.StoreToTemp(assignment.Right, out BoundAssignmentOperator boundAssignmentToTemp);
+                    temps.Add(valueTemp.LocalSymbol);
+                    sideEffects.Add(boundAssignmentToTemp);
+
+                    dictionary.Add((PropertySymbol)left.MemberSymbol, valueTemp);
+                }
+
+                var builder = ArrayBuilder<BoundExpression>.GetInstance(anonymousType.Properties.Length);
+                foreach (var property in anonymousType.Properties)
+                {
+                    if (dictionary.TryGetValue(property, out var initializerValue))
+                    {
+                        builder.Add(initializerValue);
+                    }
+                    else
+                    {
+                        // The values that are implicitly copied over will get evaluated afterwards, in the order they are needed
+                        builder.Add(_factory.Property(oldValue, property));
+                    }
+                }
+
+                dictionary.Free();
+                return builder.ToImmutableAndFree();
+            }
         }
 
         [return: NotNullIfNotNull("initializerExpressionOpt")]

src/Compilers/CSharp/Portable/Lowering/SyntheticBoundNodeFactory.cs

@@ -633,6 +633,12 @@ public BoundObjectCreationExpression New(NamedTypeSymbol type, params BoundExpre
         public BoundObjectCreationExpression New(MethodSymbol ctor, params BoundExpression[] args)
             => New(ctor, args.ToImmutableArray());
 
+        public BoundObjectCreationExpression New(NamedTypeSymbol type, ImmutableArray<BoundExpression> args)
+        {
+            var ctor = type.InstanceConstructors.Single(c => c.ParameterCount == args.Length);
+            return New(ctor, args);
+        }
+
         public BoundObjectCreationExpression New(MethodSymbol ctor, ImmutableArray<BoundExpression> args)
             => new BoundObjectCreationExpression(Syntax, ctor, args) { WasCompilerGenerated = true };