[Bug]: SwaggerGeneratorException (Failed to generate schema for type - System.Nullable`1[{enum_type}]) when using UseAllOfToExtendedReferenceSchemas

[marcominerva]

Describe the bug

If I use the UseAllOfToExtendedReferenceSchemas extension method:

builder.Services.AddSwaggerGen(options =>
{
    options.UseAllOfToExtendReferenceSchemas();
});

And then I use a nullable enum type:

[ApiController]
[Route("api/[controller]")]
public class EnumController : ControllerBase
{
    [HttpGet]
    public IActionResult Get(LogLevel? logLevel = LogLevel.Error) => Ok(new { logLevel });
}

public enum LogLevel
{
    Verbose,
    Debug,
    Information,
    Warning,
    Error,
    Fatal
}

When I run the application, I get the following exception:

SwaggerGeneratorException: Failed to generate schema for type - System.Nullable1[LogLevel]`

With this inner exception:

ArgumentException: The specified type LogLevel must derive from the specific value's type System.Int32.

Expected behavior

Swashbuckle.AspNetCore should not throw any exception.

Actual behavior

No response

Steps to reproduce

Minimal repro: https://github.com/marcominerva/NullableEnumIssue

Exception(s) (if any)

Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Failed to generate Operation for action - EnumController.Get (NullableEnumIssue). See inner exception
 ---> Swashbuckle.AspNetCore.SwaggerGen.SwaggerGeneratorException: Failed to generate schema for type - System.Nullable`1[LogLevel]. See inner exception
 ---> System.ArgumentException: The specified type LogLevel must derive from the specific value's type System.Int32.
   at System.Text.Json.ThrowHelper.ThrowArgumentException_DeserializeWrongType(Type type, Object value)
   at System.Text.Json.JsonSerializer.ValidateInputType(Object value, Type inputType)
   at System.Text.Json.JsonSerializer.Serialize(Object value, Type inputType, JsonSerializerOptions options)
   at Swashbuckle.AspNetCore.SwaggerGen.JsonSerializerDataContractResolver.JsonConverterFunc(Object value, Type type)
   at Swashbuckle.AspNetCore.SwaggerGen.JsonSerializerDataContractResolver.<>c__DisplayClass2_0.<GetDataContractForType>b__4(Object value)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchemaForParameter(Type modelType, SchemaRepository schemaRepository, ParameterInfo parameterInfo, ApiParameterRouteInfo routeInfo)
   at Swashbuckle.AspNetCore.SwaggerGen.SchemaGenerator.GenerateSchema(Type modelType, SchemaRepository schemaRepository, MemberInfo memberInfo, ParameterInfo parameterInfo, ApiParameterRouteInfo routeInfo)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateSchema(Type type, SchemaRepository schemaRepository, PropertyInfo propertyInfo, ParameterInfo parameterInfo, ApiParameterRouteInfo routeInfo)
   --- End of inner exception stack trace ---
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateSchema(Type type, SchemaRepository schemaRepository, PropertyInfo propertyInfo, ParameterInfo parameterInfo, ApiParameterRouteInfo routeInfo)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateParameter(ApiParameterDescription apiParameter, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.<>c__DisplayClass16_0.<GenerateParameters>b__1(ApiParameterDescription apiParam)
   at System.Linq.Enumerable.WhereSelectListIterator`2.ToList()
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateParameters(ApiDescription apiDescription, SchemaRepository schemaRespository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository)
   --- End of inner exception stack trace ---
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperation(ApiDescription apiDescription, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GenerateOperations(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GeneratePaths(IEnumerable`1 apiDescriptions, SchemaRepository schemaRepository)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerDocumentWithoutFilters(String documentName, String host, String basePath)
   at Swashbuckle.AspNetCore.SwaggerGen.SwaggerGenerator.GetSwaggerAsync(String documentName, String host, String basePath)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

Swashbuckle.AspNetCore version

6.6.1

.NET Version

8.0.5

Anything else?

No response

[martincostello]

Looks similar to #2885, but I think it's a different case but the same root cause.

[martincostello]

Thanks for the report, I've pushed the fix into the existing PR to fix the other issue.

[martincostello]

Thanks for reporting this issue - the fix is available in Swashbuckle.AspNetCore 6.6.2.