Adding an attribute class makes the build fail with an unrelated error

[asklar]

Describe the bug
Adding a class that derives from Attribute will cause the build to fail with an unrelated error

To Reproduce

namespace CSWinRT_AuthoringPlayground
{
    public sealed class X {}

    // Commenting the following line makes the build succeed
    public sealed class MyAttr : Attribute { }

}

Produces the error:
error CS0246: The type or namespace name 'IXClass' could not be found (are you missing a using directive or an assembly reference?)

Expected behavior
Build succeeds with proper output

Version Info
CsWinRT 1.6.4

[asklar]

the issue seems to be bad codegen from the generator, see how IntelliSense thinks the class name is global and errors out once it encounters the :: etc.:

[j0shuams]

@asklar Is this more-or-less the same issue we saw yesterday with Windows.Foundation.Point not being found?

[asklar]

@j0shuams this is a different one- this one is about the generator outputting invalid C# code