Fix #98050. (#98058)

Author: eiriktsarpalis

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

@@ -796,7 +796,7 @@ 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/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorTests.cs

@@ -795,5 +795,32 @@ public partial class MyContext : JsonSerializerContext
             CompilationHelper.RunJsonSourceGenerator(compilation);
         }
 #endif
+
+        [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);
+        }
     }
 }