[release/8.0] Fix STJ SG regression in handling of property names that are reserved keywords.

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Emitter.cs

@@ -798,7 +798,7 @@ private void GenerateFastPathFuncForObject(SourceWriter writer, ContextGeneratio
                     if (defaultCheckType != DefaultCheckType.None)
                     {
                         // Use temporary variable to evaluate property value only once
-                        string localVariableName =  $"__value_{propertyGenSpec.NameSpecifiedInSourceCode}";
+                        string localVariableName =  $"__value_{propertyGenSpec.NameSpecifiedInSourceCode.TrimStart('@')}";
                         writer.WriteLine($"{propertyGenSpec.PropertyType.FullyQualifiedName} {localVariableName} = {objectExpr}.{propertyGenSpec.NameSpecifiedInSourceCode};");
                         propValueExpr = localVariableName;
                     }

src/libraries/System.Text.Json/src/System.Text.Json.csproj

@@ -9,7 +9,7 @@
     <IncludeInternalObsoleteAttribute>true</IncludeInternalObsoleteAttribute>
     <IsPackable>true</IsPackable>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
-    <ServicingVersion>2</ServicingVersion>
+    <ServicingVersion>3</ServicingVersion>
     <PackageDescription>Provides high-performance and low-allocating types that serialize objects to JavaScript Object Notation (JSON) text and deserialize JSON text to objects, with UTF-8 support built-in. Also provides types to read and write JSON text encoded as UTF-8, and to create an in-memory document object model (DOM), that is read-only, for random access of the JSON elements within a structured view of the data.
 
 The System.Text.Json library is built-in as part of the shared framework in .NET Runtime. The package can be installed when you need to use it in other target frameworks.</PackageDescription>

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorTests.cs

@@ -737,5 +737,32 @@ public class MyClass
             Compilation compilation = CompilationHelper.CreateCompilation(source, parseOptions: CompilationHelper.CreateParseOptions(languageVersion));
             CompilationHelper.RunJsonSourceGenerator(compilation);
         }
+
+        [Fact]
+        public void FastPathWithReservedKeywordPropertyNames_CompilesSuccessfully()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/98050
+
+            string source = """
+                using System.Text.Json.Serialization;
+
+                public class Model
+                {
+                    public string type { get; set; }
+                    public string alias { get; set; }
+                    public string @class { get; set; }
+                    public string @struct { get; set; }
+                }
+
+                [JsonSourceGenerationOptions(DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull)]
+                [JsonSerializable(typeof(Model))]
+                internal partial class ModelContext : JsonSerializerContext
+                {
+                }
+                """;
+
+            Compilation compilation = CompilationHelper.CreateCompilation(source);
+            CompilationHelper.RunJsonSourceGenerator(compilation);
+        }
     }
 }