ModelBuilder throws exception during registration of key property of nullable enum type

[janreiner]

The ODataConventionalModelBuilder throws an exception durng registration of entities with a key property that is of a nullable enum type:
System.ArgumentException: Type provided must be an Enum. (Parameter 'enumType')

Although key properties are never nullable in the created model, it should be possible to register key properties of a nullable enum type since there is no error for key properties of a nullable primitve type like int?.

Assemblies affected

OData ModelBuilder 1.0.9

Reproduce steps

1. Create a new .Net 6 project
2. Have an entity with a property of a nullable enum type
3. Create an ODataConventionalModelBuilder instance
4. Register the entity type
5. Create the EdmModel

Example code:

using Microsoft.OData.ModelBuilder;
using System.ComponentModel.DataAnnotations;

var builder = new ODataConventionModelBuilder();
builder.EnableLowerCamelCase();

builder.EntityType<Customer>();
var model = builder.GetEdmModel();

enum Classification
{
    None,
    Test
}

class Customer
{
    [Key]
    public Classification? Classification { get; set; }
}

Expected result

The model builder would successfully create a model with a Customer entity that has a key property of type Classification.
Since it is a key property, it would be marked as non-nullable in the created model.

Actual result

When creating the EdmModel the following exception occurs:

Unhandled exception. System.ArgumentException: Type provided must be an Enum. (Parameter 'enumType')
   at System.RuntimeType.GetEnumUnderlyingType()
   at System.Enum.GetUnderlyingType(Type enumType)
   at Microsoft.OData.ModelBuilder.EnumTypeConfiguration..ctor(ODataModelBuilder builder, Type clrType)
   at Microsoft.OData.ModelBuilder.ODataModelBuilder.AddEnumType(Type clrType)
   at Microsoft.OData.ModelBuilder.ODataConventionModelBuilder.AddEnumType(Type type)
   at Microsoft.OData.ModelBuilder.EntityTypeConfiguration.HasKey(PropertyInfo keyProperty)
   at Microsoft.OData.ModelBuilder.Conventions.Attributes.KeyAttributeEdmPropertyConvention.Apply(StructuralPropertyConfiguration edmProperty, StructuralTypeConfiguration structuralTypeConfiguration, Attribute attribute, ODataConventionModelBuilder model)
   at Microsoft.OData.ModelBuilder.Conventions.Attributes.AttributeEdmPropertyConvention`1.Apply(TPropertyConfiguration edmProperty, StructuralTypeConfiguration structuralTypeConfiguration, ODataConventionModelBuilder model)
   at Microsoft.OData.ModelBuilder.Conventions.Attributes.AttributeEdmPropertyConvention`1.Apply(PropertyConfiguration edmProperty, StructuralTypeConfiguration structuralTypeConfiguration, ODataConventionModelBuilder model)
   at Microsoft.OData.ModelBuilder.ODataConventionModelBuilder.ApplyPropertyConvention(IEdmPropertyConvention propertyConvention, StructuralTypeConfiguration edmTypeConfiguration)
   at Microsoft.OData.ModelBuilder.ODataConventionModelBuilder.ApplyTypeAndPropertyConventions(StructuralTypeConfiguration edmTypeConfiguration)
   at Microsoft.OData.ModelBuilder.ODataConventionModelBuilder.MapTypes()
   at Microsoft.OData.ModelBuilder.ODataConventionModelBuilder.GetEdmModel()
   at Program.<Main>$(String[] args) in C:\Temp\Example\Program.cs:line 8

Additional detail

• The same exception is thrown if the property is manually registered as a key on the entity type with the HasKey-method.
• Properties of a nullable enum type that are no key can be registered on a entity type.
• The registration of key properties of a nullable primitive type like int? are successfull. The model contains a key property of the primitive type that is marked as non-nullable since it is a key.
• After a first look into the code: There might be missing a call to TypeHelper.GetUnderlyingType for the case of an enum type in the HasKey-method which is used in other code parts of the project.