Given the ability to add a number of custom conditions, it's possible for us to find an exports field like the following:

{
    "name": "test",
    "exports": {
        "a": {
            "c": "./$a.c",
        },
        "b": {
            "d": "./$b.d",
        }
    }
}

In the case that conditions a, b, and d are true, and c is not, the exports algorithm will throw an error stating No known conditions for "." specifier in "test" package. I believe that this should resolve to ./$b.d.