Extension indexers: allow and resolve extension indexers

Author: Julien Couvreur

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

@@ -91,7 +91,6 @@ internal MethodInfo ReplaceWithExtensionImplementation(out bool wasError)
                 var setMethod = replace(SetMethod);
                 Symbol symbol = ReferenceEquals(Symbol, Method) && method is not null ? method : Symbol;
 
-                Debug.Assert(SetMethod?.IsExtensionBlockMember() != true);
                 wasError = (Method is not null && method is null) || (SetMethod is not null && setMethod is null);
 
                 return new MethodInfo(symbol, method, setMethod);
@@ -634,10 +633,11 @@ private BoundIndexerAccess BindIndexerDefaultArgumentsAndParamsCollection(BoundI
         {
             var coreValueKind = valueKind & ValueKindSignificantBitsMask;
             AccessorKind accessorKind = GetIndexerAccessorKind(indexerAccess, valueKind);
-            var useSetAccessor = coreValueKind == BindValueKind.Assignable && indexerAccess.Indexer.RefKind != RefKind.Ref;
+            PropertySymbol indexer = indexerAccess.Indexer;
+            var useSetAccessor = coreValueKind == BindValueKind.Assignable && indexer.RefKind != RefKind.Ref;
             var accessorForDefaultArguments = useSetAccessor
-                ? indexerAccess.Indexer.GetOwnOrInheritedSetMethod()
-                : indexerAccess.Indexer.GetOwnOrInheritedGetMethod();
+                ? indexer.GetOwnOrInheritedSetMethod()
+                : indexer.GetOwnOrInheritedGetMethod();
             if (accessorForDefaultArguments is not null)
             {
                 var argumentsBuilder = ArrayBuilder<BoundExpression>.GetInstance(accessorForDefaultArguments.ParameterCount);
@@ -664,7 +664,7 @@ private BoundIndexerAccess BindIndexerDefaultArgumentsAndParamsCollection(BoundI
                 }
 
                 BitVector defaultArguments = default;
-                Debug.Assert(parameters.Length == indexerAccess.Indexer.Parameters.Length);
+                Debug.Assert(parameters.Length == indexer.Parameters.Length);
 
                 ImmutableArray<string?> argumentNamesOpt = indexerAccess.ArgumentNamesOpt;
 
@@ -690,9 +690,8 @@ private BoundIndexerAccess BindIndexerDefaultArgumentsAndParamsCollection(BoundI
                         }
                     }
 
-                    // Tracked by https://github.com/dotnet/roslyn/issues/78829 : caller info on extension parameter of an extension indexer will need the receiver/argument to be passed
-                    Debug.Assert(!indexerAccess.Indexer.IsExtensionBlockMember());
-                    BindDefaultArguments(indexerAccess.Syntax, parameters, extensionReceiver: null, argumentsBuilder, refKindsBuilderOpt, namesBuilder, ref argsToParams, out defaultArguments, indexerAccess.Expanded, enableCallerInfo: true, diagnostics: diagnostics);
+                    BoundExpression? extensionReceiver = indexer.IsExtensionBlockMember() ? indexerAccess.ReceiverOpt : null;
+                    BindDefaultArguments(indexerAccess.Syntax, parameters, extensionReceiver, argumentsBuilder, refKindsBuilderOpt, namesBuilder, ref argsToParams, out defaultArguments, indexerAccess.Expanded, enableCallerInfo: true, diagnostics: diagnostics);
 
                     if (namesBuilder is object)
                     {
@@ -704,7 +703,7 @@ private BoundIndexerAccess BindIndexerDefaultArgumentsAndParamsCollection(BoundI
                 indexerAccess = indexerAccess.Update(
                     indexerAccess.ReceiverOpt,
                     indexerAccess.InitialBindingReceiverIsSubjectToCloning,
-                    indexerAccess.Indexer,
+                    indexer,
                     argumentsBuilder.ToImmutableAndFree(),
                     argumentNamesOpt,
                     refKindsBuilderOpt?.ToImmutableOrNull() ?? default,

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

@@ -762,6 +762,14 @@ internal static void ReportDiagnosticsIfDisallowedExtension(BindingDiagnosticBag
             }
         }
 
+        internal static void ReportDiagnosticsIfDisallowedExtensionIndexer(BindingDiagnosticBag diagnostics, PropertySymbol property, SyntaxNode syntax)
+        {
+            if (property.IsExtensionBlockMember())
+            {
+                MessageID.IDS_FeatureExtensionIndexers.CheckFeatureAvailability(diagnostics, syntax);
+            }
+        }
+
         internal static void ReportDiagnosticsIfUnmanagedCallersOnly(BindingDiagnosticBag diagnostics, MethodSymbol symbol, SyntaxNodeOrToken syntax, bool isDelegateConversion)
         {
             var unmanagedCallersOnlyAttributeData = symbol.GetUnmanagedCallersOnlyAttributeData(forceComplete: false);

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

@@ -242,6 +242,7 @@ private ImmutableArray<Symbol> BindExtensionMemberCref(ExtensionMemberCrefSyntax
                 memberName = GetOperatorMethodName(operatorSyntax);
                 parameters = operatorSyntax.Parameters;
             }
+            // PROTOTYPE handle IndexerMemberCrefSyntax and check for LangVersion
 
             if (memberName == null)
             {
@@ -326,7 +327,8 @@ ImmutableArray<Symbol> computeSortedAndFilteredCrefExtensionMembers(NamespaceOrT
 
                     foreach (var candidate in candidates)
                     {
-                        if (!SourceMemberContainerTypeSymbol.IsAllowedExtensionMember(candidate))
+                        // PROTOTYPE we should find all the members supported in latest LangVer, but report if they require a new LangVer than current
+                        if (!SourceMemberContainerTypeSymbol.IsAllowedExtensionMember(candidate, LanguageVersion.Preview))
                         {
                             continue;
                         }