Extensions: disallow indexers (#78626)

Author: Julien Couvreur

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs

@@ -4645,8 +4645,14 @@ static void checkExtensionMember(Symbol member, BindingDiagnosticBag diagnostics
                         break;
 
                     case SymbolKind.Property:
-                        // Tracked by https://github.com/dotnet/roslyn/issues/76130 : add full support for indexers or disallow them
-                        return;
+                        if (!((PropertySymbol)member).IsIndexer)
+                        {
+                            return;
+                        }
+                        else
+                        {
+                            break;
+                        }
 
                     case SymbolKind.Field:
                     case SymbolKind.Event: