I've also been using this patch, which rebinds `this-package' within
package variants to refer instead to the variant rather than the
original.  When I tried applying a deep transformation to icecat, it
failed to reach icecat-minimal or something like that.  Or maybe it was
some input of icecat - might have been my local version of mesa that had
breakage.  Anyway, I've since forgotten exactly what happened and why (I
didn't actually write the commit message until quite some time later),
but I do know that this patch fixed it.

I might try to rediscover what the problem was later, but thought it
would be good to make you aware of this so as to hopefully consolidate
world rebuilds.

- Ulf