Fix flags enums not recognized on source-generator resolver methods

src/HotChocolate/Core/src/Types.Analyzers/Helpers/TypeReferenceBuilder.cs

@@ -174,6 +174,16 @@ private static (string TypeStructure, string TypeDefinition, string? Nullability
         }
 
         var typeName = GetFullyQualifiedTypeName(underlyingType);
+
+        if (IsFlagsEnumType(underlyingType))
+        {
+            return (
+                string.Empty,
+                typeName,
+                CreateNullabilityLiteral(underlyingType, isNullable),
+                IsSimpleType: true);
+        }
+
         var compliantTypeName = MakeGraphQLCompliant(typeName);
         var nullability = ShouldPreserveNullability(underlyingType)
             ? CreateNullabilityLiteral(underlyingType, isNullable)
@@ -328,6 +338,24 @@ private static bool IsArrayType(ITypeSymbol namedType, [NotNullWhen(true)] out I
         return false;
     }
 
+    private static bool IsFlagsEnumType(ITypeSymbol typeSymbol)
+    {
+        if (typeSymbol.TypeKind != TypeKind.Enum)
+        {
+            return false;
+        }
+
+        foreach (var attribute in typeSymbol.GetAttributes())
+        {
+            if (attribute.AttributeClass?.ToDisplayString() == WellKnownAttributes.FlagsAttribute)
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
     private static bool TryGetListElementType(
         INamedTypeSymbol namedType,
         [NotNullWhen(true)] out ITypeSymbol? elementType)

src/HotChocolate/Core/src/Types.Analyzers/WellKnownAttributes.cs

@@ -38,6 +38,7 @@ public static class WellKnownAttributes
     public const string DescriptorAttribute = "HotChocolate.Types.DescriptorAttribute";
     public const string GraphQLDeprecatedAttribute = "HotChocolate.GraphQLDeprecatedAttribute";
     public const string ObsoleteAttribute = "System.ObsoleteAttribute";
+    public const string FlagsAttribute = "System.FlagsAttribute";
     public const string BatchResolverAttribute = "HotChocolate.Types.BatchResolverAttribute";
     public const string GraphQLTypeAttribute = "HotChocolate.GraphQLTypeAttribute";
     public const string IsSelectedAttribute = "HotChocolate.Types.IsSelectedAttribute";

src/HotChocolate/Core/test/Types.Analyzers.Integration.Tests/FlagsEnumMethodBindingProbe.cs

@@ -0,0 +1,36 @@
+namespace HotChocolate.Types;
+
+[Flags]
+public enum AnimalKind
+{
+    Dog = 1,
+    Cat = 2
+}
+
+public class Animal
+{
+    public required string Id { get; set; }
+
+    public required AnimalKind Kind { get; set; }
+}
+
+public class Zoo
+{
+    public required string Id { get; set; }
+}
+
+[ObjectType<Zoo>]
+public static partial class ZooType
+{
+    public static AnimalKind GetAnimals([Parent] Zoo zoo)
+        => AnimalKind.Dog | AnimalKind.Cat;
+}
+
+public static partial class Query
+{
+    public static Animal GetAnimal()
+        => new() { Id = "1", Kind = AnimalKind.Dog | AnimalKind.Cat };
+
+    public static Zoo GetZoo()
+        => new() { Id = "1" };
+}

src/HotChocolate/Core/test/Types.Analyzers.Integration.Tests/FlagsEnumMethodBindingTests.cs

@@ -0,0 +1,71 @@
+using HotChocolate.Execution;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace HotChocolate.Types;
+
+public class FlagsEnumMethodBindingTests
+{
+    [Fact]
+    public async Task FlagsEnum_Should_CoerceToFlagsObject_When_ReturnedFromPropertyBoundField()
+    {
+        // arrange
+        var executor = await new ServiceCollection()
+            .AddGraphQLServer()
+            .AddIntegrationTestTypes()
+            .AddPagingArguments()
+            .ModifyOptions(o => o.EnableFlagEnums = true)
+            .BuildRequestExecutorAsync(cancellationToken: TestContext.Current.CancellationToken);
+
+        // act
+        var result = await executor.ExecuteAsync(
+            "{ animal { kind { isDog isCat } } }",
+            TestContext.Current.CancellationToken);
+
+        // assert
+        result.MatchInlineSnapshot(
+            """
+            {
+              "data": {
+                "animal": {
+                  "kind": {
+                    "isDog": true,
+                    "isCat": true
+                  }
+                }
+              }
+            }
+            """);
+    }
+
+    [Fact]
+    public async Task FlagsEnum_Should_CoerceToFlagsObject_When_ReturnedFromMethodBoundField()
+    {
+        // arrange
+        var executor = await new ServiceCollection()
+            .AddGraphQLServer()
+            .AddIntegrationTestTypes()
+            .AddPagingArguments()
+            .ModifyOptions(o => o.EnableFlagEnums = true)
+            .BuildRequestExecutorAsync(cancellationToken: TestContext.Current.CancellationToken);
+
+        // act
+        var result = await executor.ExecuteAsync(
+            "{ zoo { animals { isDog isCat } } }",
+            TestContext.Current.CancellationToken);
+
+        // assert
+        result.MatchInlineSnapshot(
+            """
+            {
+              "data": {
+                "zoo": {
+                  "animals": {
+                    "isDog": true,
+                    "isCat": true
+                  }
+                }
+              }
+            }
+            """);
+    }
+}