"CreateWithIdentity" not generated on base class when derived class is in separate file

[jviau]

Steps to reproduce:

A.cs

    [GenerateImmutable]
    public partial class A
    {
        readonly string field1;
    }

B.cs

    [GenerateImmutable]
    public partial class B : A
    {
        readonly string field2;
    }

This will produce an error where A.generated.cs does not have CreateWithIdentity, yet B.generated.cs will still try to call it in ToA(), causing an error.

Putting both class A and class B in the same file correctly generates the CreateWithIdentity method.

[AArnott]

Spreading type families across source files is not yet supported in general.
Closing as a child of #61, which I filed to track the overall feature request.