Should<TEnum>(this TEnum @enum)
    where TEnum : struct, Enum
    => new EnumAssertions(@enum);

// Uses ObjectAssertions
TestEnum.One.Should().HaveFlag(TestEnum.One);
TestEnum.One.Should().HaveFlag(OtherEnum.First); // compiles but fails at runtime
TestEnum.One.Should().HaveFlag(null); // compiles but fails at runtime
((TestEnum?)TestEnum.One).Should().HaveFlag(TestEnum.One);
((TestEnum?)TestEnum.One).Should().HaveFlag(OtherEnum.First); // compiles but fails at runtime
((TestEnum?)TestEnum.One).Should().HaveFlag(null); // compiles but fails at runtime

// Uses EnumAssertions
TestEnum.One.Should().HaveFlag(TestEnum.One);
TestEnum.One.Should().HaveFlag(OtherEnum.First); // compiles but fails at runtime
//TestEnum.One.Should().HaveFlag((Enum)null); // CS0453: The type 'Enum' must be a non-nullable value type in order to use it as parameter 'T'
((TestEnum?)TestEnum.One).Should().HaveFlag(TestEnum.One);
((TestEnum?)TestEnum.One).Should().HaveFlag(OtherEnum.First); // compiles but fails at runtime
//((TestEnum?)TestEnum.One).Should().HaveFlag((Enum)null); // CS0453: The type 'Enum' must be a non-nullable value type in order to use it as parameter 'T'

public class TestEnumAssertions : EnumAssertions<TestEnum, TestEnumAssertions>
{
    public TestEnumAssertions(TestEnum subject)
        : base(subject)
    {
    }
}

public static class Ext
{
    public static TestEnumAssertions Should(this TestEnum testEnum) => 
        new TestEnumAssertions(testEnum);
}

// Uses TestEnumAssertions
TestEnum.One.Should().HaveFlag(TestEnum.One);
//TestEnum.One.Should().HaveFlag(OtherEnum.First); // CS0315: There is no boxing conversion from 'OtherEnum' to 'TestEnum'
//TestEnum.One.Should().HaveFlag((Enum)null); // CS0453: The type 'Enum' must be a non-nullable value type in order to use it as parameter 'T'

// Still uses EnumAssertions
((TestEnum?)TestEnum.One).Should().HaveFlag(TestEnum.One);
((TestEnum?)TestEnum.One).Should().HaveFlag(OtherEnum.First); // compiles but fails at runtime
//((TestEnum?)TestEnum.One).Should().HaveFlag((Enum)null); // CS0453: The type 'Enum' must be a non-nullable value type in order to use it as parameter 'T'