Fix tests and address feedback

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

@@ -116,7 +116,13 @@ private sealed class Parser
 
             private readonly HashSet<TypeGenerationSpec> _implicitlyRegisteredTypes = new();
 
-            private readonly HashSet<ValueTuple<Type, DiagnosticDescriptor, string[]>> _typeLevelDiagnostics = new();
+            /// <summary>
+            /// A list of diagnostics emitted at the type level. This is cached and emitted between the processing of context types
+            /// in order to properly retrieve [JsonSerializable] attribute applications for top-level types (since a type might occur
+            /// both at top-level and in nested object graphs, with no guarantee of the order that we will see the type).
+            /// The element tuple types specifies the serializable type, the kind of diagnostic to emit, and the diagnostic message.
+            /// </summary>
+            private readonly List<(Type, DiagnosticDescriptor, string[])> _typeLevelDiagnostics = new();
 
             private JsonKnownNamingPolicy _currentContextNamingPolicy;
 
@@ -246,6 +252,11 @@ public Parser(Compilation compilation, in JsonSourceGenerationContext sourceGene
 
                 foreach (ClassDeclarationSyntax classDeclarationSyntax in classDeclarationSyntaxList)
                 {
+                    // Ensure context-scoped metadata caches are empty.
+                    Debug.Assert(_typeGenerationSpecCache.Count == 0);
+                    Debug.Assert(_implicitlyRegisteredTypes.Count == 0);
+                    Debug.Assert(_typeLevelDiagnostics.Count == 0);
+
                     CompilationUnitSyntax compilationUnitSyntax = classDeclarationSyntax.FirstAncestorOrSelf<CompilationUnitSyntax>();
                     SemanticModel compilationSemanticModel = compilation.GetSemanticModel(compilationUnitSyntax.SyntaxTree);
 
@@ -342,7 +353,7 @@ public Parser(Compilation compilation, in JsonSourceGenerationContext sourceGene
                     contextGenSpecList ??= new List<ContextGenerationSpec>();
                     contextGenSpecList.Add(contextGenSpec);
 
-                    // Clear the cache of generated metadata between the processing of context classes.
+                    // Clear the caches of generated metadata between the processing of context classes.
                     _typeGenerationSpecCache.Clear();
                     _implicitlyRegisteredTypes.Clear();
                     _typeLevelDiagnostics.Clear();

src/libraries/System.Text.Json/gen/Reflection/ReflectionExtensions.cs

@@ -49,23 +49,20 @@ private static bool HasJsonConstructorAttribute(ConstructorInfo constructorInfo)
 
         public static Location? GetDiagnosticLocation(this Type type)
         {
-            TypeWrapper? typeWrapper = type as TypeWrapper;
-            Debug.Assert(typeWrapper != null);
-            return typeWrapper.Location;
+            Debug.Assert(type is TypeWrapper);
+            return ((TypeWrapper)type).Location;
         }
 
         public static Location? GetDiagnosticLocation(this PropertyInfo propertyInfo)
         {
-            PropertyInfoWrapper? propertyInfoWrapper = propertyInfo as PropertyInfoWrapper;
-            Debug.Assert(propertyInfoWrapper != null);
-            return propertyInfoWrapper.Location;
+            Debug.Assert(propertyInfo is PropertyInfoWrapper);
+            return ((PropertyInfoWrapper)propertyInfo).Location;
         }
 
         public static Location? GetDiagnosticLocation(this FieldInfo fieldInfo)
         {
-            FieldInfoWrapper? fieldInfoWrapper = fieldInfo as FieldInfoWrapper;
-            Debug.Assert(fieldInfoWrapper != null);
-            return fieldInfoWrapper.Location;
+            Debug.Assert(fieldInfo is FieldInfoWrapper);
+            return ((FieldInfoWrapper)fieldInfo).Location;
         }
     }
 }

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

@@ -11,7 +11,6 @@
 using System.Text.Encodings.Web;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
-using Microsoft.CodeAnalysis.Text;
 using Xunit;
 
 namespace System.Text.Json.SourceGeneration.UnitTests
@@ -340,16 +339,20 @@ public partial class MyJsonContext : JsonSerializerContext
         internal static void CheckDiagnosticMessages(
             DiagnosticSeverity level,
             ImmutableArray<Diagnostic> diagnostics,
-            (TextSpan Location, string Message)[] expectedDiags)
+            (Location Location, string Message)[] expectedDiags,
+            bool sort = true)
         {
-            (TextSpan Location, string Message)[] actualDiags = diagnostics
+            (Location Location, string Message)[] actualDiags = diagnostics
                 .Where(diagnostic => diagnostic.Severity == level)
-                .Select(diagnostic => (diagnostic.Location.SourceSpan, diagnostic.GetMessage()))
+                .Select(diagnostic => (diagnostic.Location, diagnostic.GetMessage()))
                 .ToArray();
 
-            // Can't depend on reflection order when generating type metadata.
-            Array.Sort(actualDiags);
-            Array.Sort(expectedDiags);
+            if (sort)
+            {
+                // Can't depend on reflection order when generating type metadata.
+                Array.Sort(actualDiags);
+                Array.Sort(expectedDiags);
+            }
 
             if (CultureInfo.CurrentUICulture.Name.StartsWith("en", StringComparison.OrdinalIgnoreCase))
             {